604 BOTANY OF THE LIVING PLANT 



coherent or even entire blades, sometimes with stomata on the upper surface, 

 as in Nuphar. Further, in Cabomba, or in the variety of Ranunculus aquatilis 

 described as " heterophy litis, Fries," two distinct types of leaf are borne on the 

 same plant : — those submerged are dissected, those floating or subaerial being 

 lobed, or even entire. Lastly, in Aponogeton fenestralis (Juncaginaceae), a 

 denizen of still waters, the broad submerged blade is perforated ; which gives 

 as regards the surface-volume ratio, practically the same result as dissection. 

 In all such cases, provided the thickness of the leaf be not increased, the 

 elaboration of form will necessarily tend towards maintaining the surface- 

 volume ratio, notwithstanding the absence of stomata from the submerged 

 surfaces. Thus the consequent difficulty of gaseous interchange that this 

 entails in parts that are submerged tends to be overcome. 



Conclusion 



In the preceding pages limiting surfaces of the plant-body, whether 

 external or internal, have been discussed in terms of the Principle of 

 Similarity. That progressive elaborations of form and structure take 

 place as the size of the individual increases has been demonstrated on 

 a wide basis. Indeed, the existence of a Size and Form correlation 

 may now be accepted as a fact. The basic problem that awaits solu- 

 tion, and concerning which we have very little precise knowledge, is 

 that of the mechanism by which the relationship is brought about. It 

 has been seen that the adult form and structure of plants admit of 

 interpretation in terms of function. But however plausible the 

 biological advantage of each adjustment in form or structure, follow- 

 ing on change in size of plants, may seem to be, the many inter-related 

 factors, which are responsible for bringing about these adjustments, 

 are at work primarily in the apical growing region, and must be 

 studied there. 



