52 HYDROCHARITACEAE [CH. 



and partly aerial, the exposed region bore stomates, while the 

 submerged part had none. Recent work has shown that this 

 is altogether too simple an account of the position. It has been 

 demonstrated 1 , for instance, that leaves which are entirely, or 

 almost entirely submerged, may nevertheless have stomates 

 throughout their entire length. On the other hand, in the case 

 of a plant which was growing at the bottom of the water, and 

 of which the outer leaves were partly aerial, it was found that 

 these outer leaves bore no stomates whatever, but a transition 

 to stomate-bearing leaves was observed among the younger 

 leaves; the youngest, which were also the deepest in the water, 

 bore the most numerous stomates. The interpretation sug- 

 gested by the writer to whom we owe these observations, is 

 that the leaf with stomates is the higher form, which can only 

 be developed in favourable surroundings, while the stomate-free 

 leaves are primary leaves, occurring typically under conditions 

 of poor nutrition. We shall return to this subject later on, in 

 considering heterophylly in general 2 . 



Besides the epidermis, the other tissues of the leaf show 

 certain interesting features. The vascular skeleton consists of 

 five, or more, strong longitudinal veins united by transverse 

 connexions. Spirally thickened tracheids occur in the bundles 

 even in the submerged leaves. In the transverse section of the 

 rather thick lamina, besides the main row of normally orientated 

 bundles, there are two rows of small bundles, one row lying 

 near the under side and normally orientated, and the other 

 towards the upper surface and inversely orientated 3 . The occur- 

 rence of these inverted bundles in the leaves of the Hydrochari- 

 taceae is significant in connexion with the 'phyllode theory' 

 of the Monocotyledonous leaf 4 . 



In the axil of each leaf of the Water Soldier are found the 

 mucilage-secreting scales (squamulae intrav agin ales) character- 

 istic of the Helobieae 5 . 



1 Montesantos, N. (1913). 2 See pp. 156-160. 



3 Solereder, H. (1913). 4 Arber, A. (1918); see also p. 46. 



5 Nolte, E. F. (1825) and Irmisch, T. (18582). 



