Insectivorous Plants

(5) Small bits of a spinach Itaf caused in 1 hr. 20 m. increased secretion ; and after 3 hrs. 20 m. plain incurvation of the margin. The margin was well inflected after 9 hrs. 15 m., but after 24 hrs. was almost fully re-expanded. The glands in contact with the spinach became dry in 72 hrs. Bits of albumen had been placed the day before on the opposite margin of this same leaf, as well as on that of a leaf with cabbage seeds, and these margins remained closely inflected for 72 hrs., showing how much more enduring is the effect of albumen than of spinach leaves or cabbage seeds.

(6) A row of small fiayments of glass was laid along one margin of a leaf ; no effect was produced in 2 hrs. 10 m., but after 3 hrs. 25 m. there seemed to be a trace of inflection, and this was distinct, though not strongly marked, after 6 hrs. The glands in contact with the fragments now secreted mere freely than before ; so that they appear to be more easily excited by the pressure of inorganic objects than are the glands of Pin- guicula vulgar in. The above slight inflection of the margin had not increased after 24 hrs., and the glands were now beginning to dry. The surface of a leaf, near the midrib and towards the base, was rubbed and scratched for some time, but no movement ensued. The long hairs which are situated here were treated in the same manner, with no effect. This latter trial was made because I thought that the hairs might perhaps be sensitive to a touch, like the filaments of Dionaea.

(7) The flower-peduncles, sepals and petals, bear glands in general appearance like those on the leaves. A piece of a flower-peduncle was therefore left for 1 hr. in a solution of one part of carbonate of ammonia to 437 of water, and this caused the glands to change from bright pink to a dull purple colour ; but their contents exhibited no distinct aggre- gation. After 8 hrs. 30 m. they became colourless. Two minute cubes of albumen were placed on the glands of a flower- peduncle, and another cube on the glands of a sepal ; but they were not excited to increased secretion, and the albumen after two days was not in the least softened. Hence these glands apparently differ greatly in function from those on the leaves.

From the foregoing observations on Pinguicula lusi- tanica we see that the naturally much incurved mar- gins of the leaves are excited to curve still farther in- wards by contact with organic and inorganic bodies ; that albumen, cabbage seeds, bits of spinach leaves, and fragments of glass, cause the glands to secrete more freely ; — that albumen is dissolved by the secretion, and cabbage seeds killed by it ; — and lastly that matter is absorbed by the glands from the insects which are caught in large numbers by the viscid secretion. The glands on the flower-peduncles seem to have no such power. This species differs from Pin- guicala vulgaris and grandiflora in the margins of the leaves, when excited by organic bodies, being inflected to a greater degree, and in the inflection lasting for a longer time. The glands, also, seem to be more easily excited to increased secretior. by bodies not yielding soluble nitrogenous matter. In other respects, as far as my observations serve, all three species agree in their functional powers.

Ulfirularia neglecta — Structure of the bladder — The uses of the several parts • — Number of imprisoned animals — Manner of capture — The bladders cannot digest animal matter, but absorb the products of its decay — Experiments on the absorption of certain fluids by the quadrifid processes — Absorption by the glands — Summary of the observation on absorption — Development of the bladders — Utricularia vulgaris — Utricularia minor — Utricularia dandestina.

I WAS led to investigate the habits and structure of the species of this genus partly from their belonging to the same natural family as Pinguicula, but more especially by Mr. Holland's statement, that " water insects are often found imprisoned in the bladders," which he suspects " are destined for the plant to feed on." * The plants which I first received as Utricularia vulgaris from the New Forest in Hampshire and from Cornwall, and which I have chiefly worked on, have been determined by Dr. Hooker to be a very rare British species, the Utricularia neglecta of Lehni.f I subsequently received the true Utricularia vulgaris from Yorkshire. Since drawing up the following description from my own observations and those of my son, Francis Darwin, an important memoir by Prof. Cohn

High Wycombe Nat. Hist. Soc.' Eev. H. M. Wilkinson, of Bistern, July 1868, p. 5. Delpino ('Ult. for having sent me several fine Osservaz. sulla Dicogamia,' &c. lots of this species from the New Crouau as having found (1858) as to send me living plants of the crustaceans within the bladders same species from neai Penzanco on Utricularia vulgaris has appeared ; * and it has been no small satisfaction to me to find that my account agrees almost completely with that of this distin- guished observer. I will publish my description as it stood before reading that by Prof. Cohn, adding occasionally some statements on his authority.

(Ttricularia neglects.) Branch with the divided leaves bearidg bladders; about twice enlarged. Utricularia neglecta. — The general appearance of a branch (about twice enlarged), with the pinnatifid leaves bearing bladders, is represented in the above sketch (fig. 17). The leaves continually bifurcate, so that d full-grown one terminates in from twenty to thirty points. Each point is tipped by a short, straight bristle ; and slight notches on the sides of the leaves bear similar bristles. On both surfaces there are many small papillae, crowned with two hemi- spherical cells in close contact. The plants float near the surface of the water, and are quite destitute of roots, even during the earliest period of growth.* They commonly inhabit, as more than one observer has remarked to me, remarkably foul ditches.

The bladders offer the chief point of interest. There are often two or three on the same divided leaf, generally near the base ; though I have seen a single one growing from the stem. They are supported on short footstalks. When fully grown, they are nearly -i^ of an inch (2*54 mm.) in length. They are trans- lucent, of a green colour, and the walls are formed of two layers of cells. The exterior cells are poly- gonal and rather large ; but at many of the points where the angles meet, there are smaller rounded cells. These latter support short conical projections, sur- mounted by two hemispherical cells in such close apposition that they appear united ; but they often separate a little when immersed in certain fluids. The papillae thus formed are exactly like those on the surfaces of the leaves. Those on the same bladder vary much in size ; and there are a few, especially on very young bladders, which have an elliptical instead of a circular outline. The two terminal cells are transparent, but must hold much matter in solution, judging from the quantity coagulated by prolonged immersion in alcohol or ether.

* I infer that this is the case om Lentibulariacese," from the The bladders are rilled with water. They generally, but by no means always, contain bubbles of air. Ac- cording to the quantity of the contained water and air, they vary much in thickness, but are always some- what compressed. At an early stage of growth, the flat or ventral surface faces the axis or stem ; but the footstalks must have some power of movement ; for in plants kept in ruy greenhouse the ventral surface was generally turned either straight or obliquely downwards. The Kev. H. M. Wilkinson examined

( Utricularia neglecta.) Bladder j much enlarged, c, collar indistinctly seen through the walls. plants for me in a state of nature, and found this commonly to be the case, but the younger bladders often had their valves turned upwards. Tho general appearance of a bladder viewed late- rally, with the appendages on the near side alone represented, is shown in the accompanying figure (fig. 18). The lower side, where the footstalk arises, is nearly straight, and I have called it the ventral surface. The other or dorsal surface is convex, and terminates in two Ions: prolongations, formed of several rows of trells, containing chlorophyll, and bearing, chiefly on

the outside, six or seven long, pointed, multicellular bristles. These prolongations of the bladder may be conveniently called the antennae, for the whole bladder (see fig. 17) curiously resembles an entoinostracan crus- tacean, the short footstalk representing the tail. In fig. 18, the near antenna alone is shown. Beneath the two antennae the end of the bladder is slightly truncated, and here is situated the most important part of the whole structure, namely the entrance and valve. On each side of the entrance from three to rarely seven long, multicellular bristles project out-

wards ; but only those (four in number) on the near side are shown in the drawing. These bristles, to- gether with those borne by the antennae, form a sort of hollow cone surrounding the entrance. The valve slopes into the cavity of the bladder, or upwards in fig. 18. It is attached on all sides to the bladder, excepting by its posterior margin, or the lower one in fig. 19, which is free, and forms one side of the slit-like orifice leading into the bladder. This margin is sharp, thin, and smooth, and rests on the edge of a rini or collar, which dips deeply into the

bladder, as shown in the longitudinal section (fig. 20) of the collar and valve ; it is also shown at c, in fig. 18. The edge of the valve can thus open only inwards. As both the valve and collar dip into the bladder, a hollow or depression is here formed, at the base of which lies the slit-like orifice. The valve is colourless, highly transparent, flexible and elastic. It is convex in a transverse direction, but has been drawn (fig. 19) in a flattened state, by which its apparent breadth is increased. It is formed,

[longitudinal vertical section through the ventral portion of a bladder; showing valve and collar, v, valve ; the whole projection above c forms the collar; b, bifid pro- cesses ; *, ventral surface of bladder. according to Cohn, of two layers of small cells, which are continuous with the two layers of larger cells forming the walls of the bladder, of which it is evi- dently a prolongation. Two pairs of transparent pointed bristles, about as long as the valve itself, arise from near the free posterior margin (fig. 18), and point obliquely outwards in the direction of the antennae. There are also on the surface of the valve numerous glands, as [ will call them ; for they have the power of absorption, though I doubt whether they ever secrete. They consist of three kinds, which

to a certain extent graduate into one another. Thoso situated round the anterior margin of the valve (upper margin in fig. 19) are very numerous and crowded together ; they consist of an oblong head on a long pedicel. The pedicel itself is formed of an elongated cell, surmounted by a short one. The glands towards the free posterior margin are much larger, few in number, and almost spherical, having short footstalks ; the head is formed by the confluence of two cells, the lower one answering to the short upper cell of the pedicel of the oblong glands. The glands of the third kind have transversely elongated heads, and are seated on very short footstalks ; so that they stand parallel and close to the surface of the valve ; they may be called the two-armed glands. The cells form- ing all these glands contain a nucleus, and are lined by a thin layer of more or less granular protoplasm, the primordial utricle of Mohl. They are filled with fluid, which must hold much matter in solution, judging from the quantity coagulated after they have been long immersed in alcohol or ether. The depres- sion in which the valve lies is also lined with innu- merable glands; those at the sides having oblong heads and elongated pedicels, exactly like the glands on the adjoining parts of the valve.

The collar (called the peristome by Cohn) is evi- dently formed, like the valve, by an inward projection of the walls of the bladder. The cells composing the outer surface, or that facing the valve, have rather thick walls, are of a brownish colour, minute, very numerous, and elongated ; the lower ones being divided into two by vertical partitions. The whole presents a complex and elegant appearance. The cells forming the inner surface are continuous with those over the whole inner surface of the bladder. The space be-

tvveen the inner and outer surface consists of coarse cellular tissue (fig. 20). The inner side is thickly covered with delicate bifid processes, hereafter to be described. The collar is thus made thick ; and it is rigid, so that it retains the same outline whether the bladder contains little or much air and water. This is of great importance, as otherwise the thin and flexible valve would be liable to be distorted, and in this case would not act properly.

Altogether the entrance into the bladder, formed by the transparent valve, with its four obliquely project- ing bristles, its numerous diversely shaped glands, surrounded by the collar, bearing glands on the inside and bristles on the outside, together with the bristles borne by the antenna, presents an extra- ordinarily complex appearance when viewed under the microscope. We will now consider the internal structure of the bladder. The whole inner surface, with the exception of the valve, is seen under a moderately high power to be covered with a serried mass of processes (fig. 21). Each of these consists of four divergent arms ; whence their name of quadrifid processes. They arise from small angular cells, at the junctions of the angles of the larger cells which form the interior of the bladder. The middle part of the upper surface of these small cells projects a little, and then contracts into a very short and narrow footstalk which bears the four arms (fig. 22). Of these, two are long, but often of not quite equal length, and project obliquely inwards and towards the posterior end of the bladder. The two others are much shorter, and project at a smaller angle, that is, are more nearly horizontal, and are directed towards the anterior end of the bladder. These arms are only moderately sharp ; they are composed of ex-

tremely thin transparent membrane, so that they can be bent or doubled in any direction without being broken. They are lined with a delicate layer of proto- plasm, as is likewise the short conical projection from which they arise. Each arm generally (but not in- variably) contains a minute, faintly brown particle, either rounded or more commonly elongated, which exhibits incessant Brownian movements. These par- Small portion of inside of blad- der, much enlarged, showing quud- rind processes.

tides slowly change their positions, and travel from one end to the other of the arms, but are commonly found near their bases. They are present in the quad- rinds of young bladders, when only about a third of their full size. They do not resemble ordinary nuclei, but I believe that they are nuclei in a modified con- dition, for when absent, I could occasionally just dis- tinguish in their places a delicate halo of matter, including a darker spot. Moreover, the quadrifids of Utricularia montana contain rather larger and much

more regularly spherical, but otherwise similar, par- ticles, which closely resemble the nuclei in the cells forming the walls of the bladders. In the present case there were sometimes two, three, or even more, nearly similar particles within a single arm ; but, as we shall hereafter see, the presence of more than one seemed always to be connected with the absorption of decayed matter. The inner side of the collar (see the previous fig. 20) is covered with several crowded rows of processes, dif- fering in no important respect from the quadrifids, except in bearing only two arms instead of four ; they are, however, rather narrower and more delicate. I shall call them the bifids. They project into the bladder, and are directed towards its posterior end. The quad- rifid and bifid processes no doubt are homologous with the papillae on the outside of the bladder and of the leaves; and we shall see that they are de- veloped from closely similar papillae.

Tlie Uses of the several Parts. — After the above long but necessary description of the parts, we will turn to their uses. The bladders have been supposed by some authors to serve as floats ; but branches which bore no bladders, and others from which they had been removed, floated perfectly, owing to the air in the intercellular spaces. Bladders containing dead and captured animals usually include bubbles of air, but these cannot have been generated solely by the pro- cess of decay, as I have often seen air in young, clean, and empty bladders ; and some old bladders with much decaying matter had no bubbles.

The real use of the bladders is to capture small aquatic animals, and this they do on a large scale. In the first lot of plants, which I received from the NVw Forest early in July, a large proportion of the fullv grown bladders contained prey ; in a second lot, re- ceived in the beginning of August, most of the bladders were empty, but plants had been selected which had grown in unusually pure water. In the first lot, my son examined seventeen bladders, in- cluding prey of some kind, and eight of these con- tained entomostracan crustaceans, three larvae of in- sects, one being still alive, and six remnants of animals so much decayed that their nature could not be distinguished. I picked out five bladders which seemed very full, and found in them four, five, eight, and ten crustaceans, and in the fifth a single much elongated larva. In five other bladders, selected from containing remains, but not appearing very full, there were one, two, four, two, and five crustaceans. A plant of Utricularia vulgaris, which had been kept in almost pure water, was placed by Cohn one evening into water swarming with crustaceans, and by the next morning most of the bladders contained these animals entrapped and swimming round and round their prisons. They remained alive jbr several days ; but at last perished, asphyxiated, as I suppose, by the oxygen in the water having been all consumed. Freshwater worms were also found by Cohn in some bladders. In all cases the bladders with decayed remains swarmed with living Algae of many kinds, Infusoria, and other low organisms, which evidently lived as intruders.

Animals enter the bladders by bending inwards the posterior free edge of the valve, which from being highly elastic shuts again instantly. As the edge is extremely thin, and fits closely against the edge of the collar, both projecting into the bladder (see section, fig. 20), it would evidently be very difficult for any animal to get out when once imprisoned, and apparently they never do escape. To show how closely the edge fits, I may mention that my son found a Daplmia whicn had inserted one of its antennoe into the slit, and it was thus held fast during a whole day. On three or four occasions I have seen long narrow larvae, both dead and alive, wedged between the corner of the valve and collar, with half their bodies within the bladder and half out.

As I felt much difficulty in understanding how such minute and weak animals, as are often captured, could force their way into the bladders, I tried many experiments to ascertain how this was effected. The free margin of the valve bends so easily that no resistance is felt when a needle or thin bristle is inserted. A thin human hair, fixed to a handle, and cut off so as to project barely % of an inch, entered with some difficulty ; a longer piece yielded instead of entering. On three occasions minute par- ticles of blue glass (so as to be easily distinguished) were placed on valves whilst under water; and on trying gently to move them with a needle, they disap- peared so suddenly that, not seeing what had happened, I thought that I had flirted them off; but on ex- amining the bladders, they were found safely enclosed. The same thing occurred to my son, who placed little cubes of green box-wood (about -^ of an inch, -423 mm.) on some valves ; and thrice in the act of placing them on, or whilst gently moving them to another spot, the valve suddenly opened and they were en- gulfed. He then placed similar bits of wood on other valves, and moved them about for some time, but they did not enter. Again, particles of blue glass were placed by me on three valves, and extremely minute shavings of lead on two other valves ; after 1 or 2 hrs. none had entered, but in from 2 to 5 hrs. all five were enclosed. One of the particles of glass was b

long splinter, of which one end rested obliquely on the valve, and after a few hours it was found fixed, half within the bladder and half projecting out, with the edge of the valve fitting closely all round, except at one angle, where a small open space was left. It was so firmly fixed, like the above mentioned larvae, that the bladder was torn from the branch and shaken, and yet the splinter did not fall out. My son also placed little cubes (about ^V of an inch, '391 mm.) of green box-wood, which were just heavy enough to sink in water, on three valves. These were examined after 19 hrs. 30 in., and were still lying on the valves ; but after 22 hrs. 30 in. one was found enclosed. I may here mention that I found in a bladder on a naturally growing plant a grain of sand, and in another bladder three grains ; these must have fallen by some accident on the valves, and then entered like the particles of glass.

The slow bending of the valve from the weight of particles of glass and even of box- wood, though largely supported by the water, is, I suppose, analogous to the slow bending of colloid substances. For instance, particles of glass were placed on various points of narrow strips of moistened gelatine, and these yielded and became bent with extreme slowness. It is much more difficult to understand IIOAV gently moving a particle from qne part of a valve to another causes it suddenly to open. To ascertain whether the valves were endowed with irritability, the surfaces of several were scratched with a needle or brushed with a fine camel-hair brush, so as to imitate the crawling move- ment of small crustaceans, but the valve did not open. Some bladders, before being brushed, were left for a time in water at temperatures between 80° and 130° F. (26°-6— 54°-4 Cent.), as, judging from a wide-

spread analogy, this would have rendered them more sensitive to irritation, or would by itself have excited movement ; but no effect was produced. We may, therefore, conclude that animals enter merely by forcing their way through the slit-like orifice ; their heads serving as a wedge. But I am surprised that such small and weak creatures as are often captured (for instance, the nauplius of a crustacean, and a tardi- grade) should be strong enough to act in this manner, seeing that it was difficult to push in one end of a bit of a hair £ of an inch in length. Nevertheless, it is certain that weak and small creatures do enter, and Mrs. Treat, of New Jersey, has been more suc- cessful than any other observer, and has often wit- nessed in the case of Utricularia clandestine!, the whole process.* She saw a tardigrade slowly walk- ing round a bladder, as if reconnoitring ; at last it crawled into the depression where the valve lies, and thru easily entered. She also witnessed the entrap- ment of various minute crustaceans. Cypris "was " quite wary, but nevertheless was often caught. " Coming to the entrance of a bladder, it would sorne- " times pause a moment, and then dash away ; at " other times it would come close up, and even ven- " ture part of the way into the entrance and back out " as if afraid. Another, more heedless, would open " the door and walk in ; but it was no. sooner in than " it manifested alarm, drew in its feet and antenna?, and closed its shell." Larvae, apparently of gnats, when "feeding near the entrance, are pretty certain " to run their heads into the net, whence there is no " retreat. A large larva is sometimes three or four ' hours in being swallowed, the process bringing to

" mind what I have witnessed when a small snake ''• makes a large frog its victim." But as the valve does not appear to be in the least irritable, the slow swallowing process must be the effect of the onward movement of the larva. It is difficult to conjecture what can attract so many creatures, animal- and vegetable-feeding crustaceans, worms, tardigrades, and various larvse, to enter the bladders. Mrs. Treat says that the larvae just referred to are vegetable-feeders, and seem to have a special liking for the long bristles round the valve, but this taste will not account for the entrance of animal- feeding crustaceans. Perhaps small aquatic animals habitually try to enter every small crevice, like that between the valve and collar, in search of food or pro tection. It is not probable that the remarkable trans- parency of the valve is an accidental circumstance, and the spot of light thus formed may serve as a guide. The long bristles round the entrance ap- parently serve for the same purpose. I believe that this is the case, because the bladders of some epi- phytic and marsh species of Utricularia which live embedded either in entangled vegetation or in mud, have no bristles round the entrance, and these under such conditions would be of no service as a guide. Nevertheless, with these epiphytic and marsh species, two pairs of bristles project from the surface of the valve, as in the aquatic species ; and their use pro- bably is to prevent too large animals from trying to force an entrance into the bladder, thus rupturing the orifice.