222 



KNOWLEDGE 



[OOTOBEK 1, 1898. 



THE AFFINITIES OF FLOWERS. 



THE BLADDERWORT AND ITS RELATIVES. 



By Felix Oswald, b.a., b.sc. 



IN wandering over some desolate moor in July or 

 August, we may perchance find a peaty pool aglow 

 with strange yellow flowers, somewhat like snap- 

 dragon, on slender stalks which rise from a green 

 feathery mass floating just beneath the surface of 

 the water. Let us lift out the whole plant and examine 

 it more closely ; we can then clearly see why it has been 

 named the bladderwort, for we find numbers of minute 

 pale-green bladders interspersed among the branching 

 feathery leaves. Observation alone will lead us to infer 

 that these little bladders are merely modified leaflets, for 

 they are set on stalks arising from the much divided leaves, 

 generally from a point of bifurcation. Moreover, they 

 still bear branching bristles at the apex, similar to the 

 leaf filaments. Some species indeed, (<'.;/., Utricularia 

 intermedia and U. i/ra liana), reach a further stage of 

 specialization in having the bladders restricted to separate 

 branches. 



There is a total absence of roots, just as in many other 

 floating plants, such as the water-fern (Sdlrinia). Indeed, 

 the rootless condition has become so deeply impressed on 

 the constitution of the bladderwort that not even a pri- 

 mary root is developed when the embryo germinates. The 

 hair-like character of the leaves may perhaps be due to 

 their having to adopt the function of roots in absorbing 

 the nutrient salts contained in solution in the water. 

 But it may be also due (as Grant Allen has suggested in 

 regard to the submerged leaves of the water crowfoot) to 

 the necessity for searching out, so to speak, for the scanty 

 amount of carbonic acid in the stiU waters frequented by 

 these plants. 



The bladders, however, form the chief interest of this 

 strange plant, for they have become differentiated into the 

 most efficient traps for catching small water animals. 

 The entrance to the bladder is firstly protected, by means 

 of branched bristles, from larger creatures, which might 

 damage the apparatus. When once past this chi'raH.r de 

 frise, an entrance is easily effected by pushing inwards the 

 elastic valve or upper lip, which closes tightly upon the 

 lower lip — a thickened cushion of tissue. But no return 

 is possible when the door has closed, and all hope of 

 freedom must be abandoned. Here the prisoners remain 

 in their dungeon until they die from suffocation or inanition, 

 victims to their own curiosity. Death usually overtakes 

 them in about twenty-four hours, although they sometimes 

 linger on for as long as six days. A close scrutiny of the 

 bladders will usually reveal a variety of small crustaceans 

 such as water fleas {Daphnia, Cypris, and Cycl/yis), larvfe of 

 gnats and midges, innumerable infusoria and diatoms, and 

 even small worms. The bladderwort, however, is not 

 always left in undisputed possession of its prey, for a water 

 spider sometimes finds it a profitable undertaking to spin 

 its silken silvery bell among the branches of the plant, and 

 to rifle the contents of the bladders. 



No digestive ferment is secreted in these traps as on the 

 leaves of the carnivorous sundew and butterwort, but we 

 find special absorbent hairs arranged in groups of four, 

 studded at intervals all over the inner surface of the bladder. 

 A gradual transition may be observed between these peculiar 

 hairs and those outside,^' which secrete a kind of mucilage, 



* Chodat has shown that these bail's arise from cells which in 

 laud-plants would have become stomata; a change of habitat necessi- 

 tating a change of function. 



perhaps attractive to the deluded visitors. It is considered 

 probable that the nitrogenous products of decomposition 

 are taken up by the internal hairs into the system of the 

 plant for assimilation — a distinct advantage for the species, 

 since peaty soils are well known to be deficient in 

 nitrogen, which is so important an element of animal and 

 plant life. It is also possible that the carbon dioxide 

 exhaled by the animals during their imprisonment may be 

 of considerable service to the plant. 



On the approach of winter the whole plant decays, with 

 the exception of the terminal bud, which is wrapped up 

 and protected by leaves closely crowded together, but 

 without any bladders. This resting bud eventually sinks 

 to the bottom of the pool, just as the frog-bit (/ii/i/roc/mr/s) 

 and many other water plants. The warmth of spring 

 rouses the dormant bud into activity, the leaves expand, 

 the stem grows, bladders are again developed in place of 

 leaflets, and the plant rises to the surface of the water. 

 Formerly it was considered that the sole reason for the ex- 

 istence of the bladders was to raise the plant from the bottom 

 of the pool after the long winter rest, and to buoy it up so as 

 to float in the most suitable position. It is possible, indeed. 



Bladderwort plant in flower, one-third le«3 than natural size. 



that their primary function was hydrostatic, and that the 

 habit of catching animals is secondary, and has induced 

 several modifications in structure ; yet it is clear that at 

 the present time the bladders cannot act merely as buoys, 

 in view of the fact that the small British Utricuhtria inter- 

 media does not float at all, but creeps along the bottom of 

 pools, anchored to the soft mud by the bladders, which, in 

 this case, are borne on separate branches of the stem. 

 Moreover, there are many purely terrestrial spacies of 

 bladderwort in the tropics which possess bladders essen- 

 tially similar to those of our aquatic species, although very 

 much smaller. They frequent, however, damp places, in 

 association with mosses and liverworts. A strange instance 

 of dependence of one plant upon another is afforded by a 

 Brazilian species [Utricularia jieliimhifoUa) \ it lives in the 

 reservoirs of water formed by theleaf rosettes of TiUandsia 

 plants (allies of the pineapple). This bladderwort spreads 

 abundantly, sending out long runners which grope their 

 way to another water receptacle of a Tillandsia, and even 

 to those cf neighbouring plants. 



The bladderwort belongs to the small family Lenti- 

 bulariaceffi, represented in Britain by only one other genus, 

 viz., the insect-catching butterwort, which is not, however, 

 a very close relative. The characteristics which they possess 

 in common show a considerable degree of specialization ; 

 for instance, both calyx and corolla are irregular, with two 

 broad lips, somewhat like a flattened snapdragon. The 

 corolla is produced into a honey-containing spur ; the 

 stamens have become reduced to two ; the ovary is uni- 

 locular {i.e., it consists of a single chamber), with the 

 ovules arranged on a central pillar ; the fruit is a capsule 



