522 REV. GEORGE HEN SLOW ON k THEORETICAL 



anil floating forms of Polygonum amphibium, which run to the 

 margin, as is usual in aquatic leaves, without any inarching. Reti- 

 culations occur abundantly between the veins. The origin of 

 this type of pinnately-nerved leaves seems to be as follows : — 

 Commencing with that of Ouvirandra, there are several ribs 

 united by cross-bars. The same arrangement occurs in the lily 

 of the valley ; but in other cases, as in Smilax, the ribs become 

 reduced to three, the cross-bars becoming longer and closer 

 together. Now, if we suppose the lateral ribs to be arrested, the 

 pinnated venation of Dieffenbachia results. 



That something like this has taken place seems probable from 

 the converse process, which occurs in the leaves of Polygonum 

 amphibium ; for while, as stated, the aerial and floating leaves 

 have feather-like veins, the submerged ones have nearly parallel 

 and longitudinal ribs, many of the lateral veins now assuming 

 the character of cross-bars. 



* 



Cause of Marginal Weakness in the Leates of Endo- 



gens. — The above brief epitome of the leaf-structure of endogens 

 will, perhaps, help one to explain another feature not infre- 

 quently observable in endogens. The inarching process at the 

 margins of the leaves of exogens, often coupled with a marginal 

 rib, aifords great strength against laceration by the wind. In 

 endogens, however, not only is the inarching usually absent, but 

 the ribs and veins are comparatively weaker, especially if they 

 be of aquatic and subaquatic plants. The facility of tearing, 

 which may be seen in the submerged leaves otNymphcea, in those 



of bananas, in palm-leaves, and blades of wheat, &c, may be thus 



traceable to an enfeebled structure primarily due to an aquatic 

 habit. 



Origin of Fenestration. — Tet another feature may receive 

 a similar interpretation. Oue effect of a watery medium is to 

 arrest the formation of the cellular tissue between the ribs and 

 veins. Hence arises the dissected type of submerged leaves of 

 many exogens, and the fenestrated condition of species of Pot a- 

 mogeton and, above all, of Ouvirandra fenestralis. 



Now, if my contention be true that aquatic plants are thus 

 descended iroin terrestrial, and can subsequently recover their 

 lost habits, by becoming terrestrial again, it would be in accord- 

 ance with all analogy that while doing so they would retain 

 aquatic features associated with aerial and terrestrial adaptations. 



