520 HEY. GEORGE HEXSLOW OX A THEORETICAL 



Ranunculus Flammula and JR. Lingua the long linear-lanceolate 

 aerial leaves have already acquired an approximation to the 

 characters of the submerged leaves of endogens ; i. e. so far as is 

 shown by the general equality of the strength of the ribs, by the 

 endeavour to place them parallel to one another, though they 

 still issue from the midrib, and by cross-bars now assuming a 

 regularly parallel position, all inclining at about 45° to the ribs. 

 Similarly in Polygonum amphibium the hairy terrestrial leaves are 

 elongated, with a comparatively strong midrib ; the numerous 

 lateral veins are pinnately arranged, with inarching margins. The 

 blade of the hairless # floating form is much shorter, with a weaker 

 midrib, but the lateral veins are similar. The venation of the 

 thin submerged leaf is quite different. It has the midrib reduced 

 to the same size as the lateral ones, which now run as parallel as 

 possible, starting at a very acute angle from near the base. The 

 marginal inarching is becoming lost. 



Of submerged linear leaves of exogens, Lobelia Dortmanni has 

 a ribbon-like leaf, which in form exactly resembles a submerged 

 endogenous leaf; but an inspection of the venation shows at once 

 that it is an adaptation from a reticulated exogenous type. As 

 in the preceding, the ribs are as parallel as possible, while irre- 

 gular reticulations are retained. 



A still further degradation is seen in Limosella aquatica. The 

 leaves of this plant have long petioles with spathulate, linear, or 

 bluntly lanceolate blades. In these the ribs run parallel in the 

 petiole and are curved in the blade, without cross-bars. 



CallifricJie and Hippuris, as already stated, appear to have 

 reverted to an almost cotyledonary type of leaf, assuming the 

 rudimentary character of venation as usually seen in the coty- 

 ledons of exogens. . 



The leaves of terrestrial endogens illustrate the retention of 

 the ancestral aquatic characters of a parallel or curvinerved ve- 

 nation more or less perfectly ; but they have superadded to this 

 other features — as reticulations, stomata, &c. — which are adap- 

 tations to an aerial existence. 



On the other hand, aquatic exogens, when their leaves become 

 submerged, retain more or less the aerial types of venation, but 

 adapt their structure to an aquatic existence ; thus renderin 

 them more or less comparable with existing endogenous leaves. 



* The almost universally hairless condition of endogens is another general 

 coincidence with the glabrous state of true water-plants. 





