INFLUENCE OF EXTERNAL STIMULI. THE MEDIUM 261 



are not so great as appear in R. fluitans, which is one of the species 

 whose adaptation is chiefly to an aquatic life. Ranunculus multifidus, 

 which stands very near the land-species of the genus, illustrates a direct 

 influence upon leaf-form its submerged leaves are much more finely 

 divided than the usual land-leaves (Fig. 128), and show in that way 

 an approach to the behaviour of the characteristically water-species l . 



In many of the Bryophyta moisture plays a part in the formation 

 of organs. Frullania dilatata 

 possesses cap-like formations 

 formed by invaginations of 

 leaf-lobes which serve the plant, 

 living as it does upon the bark 

 of trees, as capillary water- 

 reservoirs. Their formation can 

 be hindered by continuous cul- 

 ture in moisture ; then the leaf- 

 lobes remain flat 2 . Bryum 

 argentum owes its silvery sheen 

 to the death of the upper por- 

 tions of the leaves which, as 

 a non-living envelope, protect 

 the buds against great drought ; 

 if the plant be cultivated in 

 moisture the leaves remain 

 green 3 . Likewise in many 

 mosses we find that the hair-points on their leaves, which are present 

 in plants growing upon dry sunny spots, and which there serve as pro- 

 tecting tufts to the stem-buds, are not developed when the plants are 

 grown in moist places or in water. 



FiG. 128. Ranunculus multifidus. L, land-leaf. 



leaf. 



water- 



B. THE INDUCTION OF RESTING STATES THROUGH DROUGHT. 



In some Thallophyta drought causes the formation of resting 

 states the appearance of which is often associated with a profound 

 change in the whole formation of organs. Thus in Botrydium granu- 

 latum 4 , one of the Siphonieae, which consists of a bladder-like green 

 upper portion and a branched root-like subterranean portion, if it is 

 subjected to drought or strong insolation, the protoplasm wanders into 

 the rooting portion and breaks up into a number of cells which, under 



1 Regarding the complex relationship of Ranunculus aquatilis, see Goebel, Pflanzenbiologische 

 Schilderungen, ii. p. 309. 



2 Goebel, Pflanzenbiologische Schilderungen, i. fig. 76. 3 See Flora, 1896, p. 10. 

 4 See Rostafinski uncl Woronin, Uber Botrydium granulatum, in Botan. Zeitung 1877, 649. 



