104 LIFE : OUTLINES OF GENERAL BIOLOGY 



the minute leaf-blade meet at an angle of about 60°, but after a 

 certain age they are all practically closed. Each half-blade shows 

 (i) marginal bristles, (2) a glandular zone with quadrifid hairs which 

 probably secrete attractive mucus, (3) a non-glandular zone, and 

 (4) an innermost zone with digestive glands and hinged sensitive 

 bristles. These seem to require rejx^ated touches, such as a restless 

 water-tlea might well give, before they evoke the full reaction. 

 Within the closed traplet a gas-bubble is formed and absorption of 

 the digested fluid goes on slowly. 



Hl.addkkwokts. -In marshy lochs and mountain-tarns in 

 Hritain there is a widespread occurrence of the Common Bladder- 

 wort {Vtricularia vnb^aris), conspicuous for a month or two in 

 summer, when from the floating stem there rises for several inches 

 a slender flower-stalk with somewhat orchid-like golden blossoms, 

 which apjxar but once in several years. At other times the bladder- 

 wort is anything but conspicuous, for it floats in the water and its 

 leaves are small. Like some other thoroughly aquatic plants, e.g. 

 Salvinia, it is rootless, a peculiarity which may be correlated with 

 the absorptive jwwer of the whole submerged surface, rather than 

 with the carnivorous habit. 



The bladderwort belongs to the order Lentibulariacea?, which 

 also includes the butterwort, and there arc about 250 species 

 repre.senting the genus in all parts of the world. But many of these 

 are terrestrial, and a number are epiphytic. 



With the single exception of the epiphytic Vtricularia neottioides, 

 the bladderworts have many leaflets transformed into pinhead-like 

 "bladders" or traps. Each is a hollow chamber, about iV inch 

 in longest diameter, entered by a transparent valve which opens 

 inwards only, and allows of no egress, for it shuts instantly, as 

 if with a spring, against a thickened collar around the mouth. 

 Six or seven stiff hairs project from the o|x*ning into the water, and 

 secrete an attractive mucus mixed with a little sugar. It may be 

 mentioned that the sujx'rficial secretion of mucus is common in 

 submerged water-plants. 



It has Ix'en shown exjxTimentally that the secretion of the hairs 

 is attractive to small crustaceans, })opularly known as water- fleas, 

 some belonging to the order of Copeixjds, while others, e.g. the very 

 common Cypris, are Ostracods. This difference in diet corresponds 

 t(» differences in the habitat of the species; thus U. vulgaris, which 

 floats near the surface, is a Copepod-catcher, whereas U. intermedia, 

 which is submerged near the substratum, is an Ostracod-catcher. 

 In some cases there is a capture of other small animals, but the 

 restliss exploratory movements of the water-fleas make them very 

 likely booty. It is probable that they touch the yielding door of the 

 n 1-trap-like bladder by chance; in any case, there is no doubt that 

 they ( nter and are imj)risoned. The slime probably abets suffocation, 



