ABSORPTION AND MOVEMENT OF WATER 129 



the rate but, as we have already seen (page 83), the kind 

 of activity in accordance with certain stimuli. On the hairs 

 of Drosera, and on the leaves of other carnivorous plants, 

 the sugary secretion which is used for attracting and cap- 

 turing prey may be more like that formed by nectaries 

 (page 126) — due, so far as the water is concerned, to os- 

 motic suction rather than to active pressing out. The fill- 

 ing up of the pitchers of the pitcher-plants is much more 

 likely to result from active excretion of water. 



The mechanics of the secretions commonly taking place on 

 the surface of stigmas are probably identical with those of 

 nectaries, although it must be seen that the first secretion of 

 sugar, and of the water which carries it, may in both cases 

 be accomplished by active pressing out of these substances 

 by the living cells forming the surface of the gland. When 

 sap-pressure is high in the body of the plant or in the root- 

 hairs themselves, the dissolved substances passing out 

 through root-hairs may be pressed out mechanically by the 

 vital activity of the living protoplasm, as well as by the 

 difference in the osmotic pressures within and without 

 the cell (page 125). 



The substances passing out exosmotically or by other 

 pressure from the cells are seldom, if ever, such as con- 

 tribute directly to the formation of protoplasm. Proteids, 

 albuminoids, and the like, remain in the cells in spite of 

 the differences in proportional composition of the liquids 

 within and without the cells. Though some of these sub- 

 stances may be soluble, none is freely diffusible. According 

 to the differences in diffusibility, soluble substances have 

 been divided into crystalloids and coUoids, respectively sub- 

 stances readily and tardily diffusing. It is now known, con- 

 trary to former supposition, that colloids may crystallize. 

 For this reason the names are inapt and misleading. The 

 hypothetical explanation of the feeble diffusibility of colloi- 

 dal substances is this : the molecules of these highly complex 

 compounds are so large (e. g. egg-albumen, which, according 

 to Lieberkuhn, may have the formula C^HjjjNs^OjeSj*) that 



* Loew and Bokomy. Die chemisehe Kraftquelle im lebenden Proto- 

 plasma. Munich, 1882. 

 9 



