270 TRANSPIRATION CURRENT [CH. 



phyllum and Utricularia, we are still far from understanding the 

 mechanism of absorption and elimination. Here the liquid 

 exchange presumably takes place entirely by means of osmosis 

 and diffusion. But it should be noted that in both these un- 

 related genera, which are characterised by the total absence of 

 true roots, there is a tendency to the production of subterranean 

 shoots, which perform the function of roots 1 . This modification 

 of other organs for subterranean work, appears to suggest that, 

 in the course of evolution, some disadvantage has followed the 

 reduction and ultimate loss of the root system, and that an 

 attempt has been made to replace it. 



The insectivorous habit of Utricularia may also perhaps be 

 correlated with the reduction of the transpiration stream, and 



FIG. 164. Hydrocleis nymphoides, Buchen. T.S. leaf passing through the middle of 

 the apical cavity which remains roofed in with cuticle. [Sauvageau, C. (1893).] 



the consequent limitation of the food supply 2 . This is rendered 

 more probable when it is remembered that the only other car- 

 nivorous genus among water plants, Aldrovandia, is also sub- 

 merged and rootless. The resemblance of the two genera, in 

 these respects, is the more remarkable since they belong to 

 widely separated cycles of affinity. Their common insectivorous 

 habit seems to indicate that a plant, which has dispensed with 

 an active transpiration stream, needs some compensation for 

 the loss of food materials involved. 



In the present chapter, stress has been laid upon the diffi- 

 culties besetting a submerged plant in connexion with the 

 maintenance of a transpiration stream. In conclusion we must 

 glance for a moment at an embarrassment incurred by such 



1 See pp. 88, 89, 96, 97, 2 Cohn, F. (1875). 



