THE RELATION OF SIZE AND FORM IN PLANTS 603 



ment of all is based on secondary cambial activity, as in the Gymno- 

 sperms and Dicotyledons. This provides an automatic increase, not 

 only in the channels of supply but also in tissues of mechanical resis- 

 tance. Theoretically there is no limit to growth in photosynthetic 

 plants, based on continued embryology, combined with a geometrical 

 ratio of branching, internal ventilation, and cambial increase. But 

 actually there are spatial and mechanical checks on this ambitious 

 scheme. The final check of all for subaerial plants is mechanical 

 inability to support an indefinitely increasing load of stems, branches 

 and leaves, and to resist the impact of winds. Even automatic 

 cambial increase does not suffice. Overstepping the limit of resistance 

 results in fallen trees, and stripped branches, twigs, and leaves. These 

 mark various degrees of failure under the final test of mechanical 

 stress. 



Adaptation of Form in Submerged Parts. 



Chapters V. and VIII. have shown how the internal tissues of the larger 

 Plants of the Land are ventilated by a system of intercellular spaces, which 

 open through the pores of the stomata and through porous lenticels. Thus 

 they provide for that gaseous interchange with the open air that is essential 

 for the subaerial life of Land Plants. This has been fully discussed by Haber- 

 landt as illustrating the Principle of Maximum Exposure laid down in his 

 Physiological Plant Anatomy (Engl. Ed., p. 276, etc.). The effect will be 

 proportionate to the area of the internal surfaces thus exposed. 



A contrast to the condition thus seen in the vast majority of subaerial plants 

 is presented by those in which the green leaves are submerged wholly or in part. 

 Since stomata are absent from such surfaces as face the water the ventilating 

 system is sealed up within the investing epidermis. In them gaseous inter- 

 change can be conducted only by the slower process of diffusion through the 

 unbroken outer surface. The sum of its activity will then depend upon the 

 surface-volume ratio of the plant as a whole. This physiological problem faces 

 all submerged green plants : it is in fact one of size and form : and the success 

 of each individual will depend upon the measure in which that ratio is main- 

 tained. As we have seen, any elaboration of a simpler to a more complex form 

 tends to increase it. A few familiar examples may serve to illustrate how by 

 modification of form of the submerged parts the surface- volume ratio tends to 

 be upheld. It is usually by disintegration of the leaf-blade, such as is seen in 

 isolated genera and species belonging to diverse distinct families. These 

 plants suggest that they are specially adapted to meet the physiological 

 demands consequent on a submerged habit, and the absence of stomata. 



Such examples are presented in varying degree by the many different forms of 

 Ranunculus aquatilis Linn, (Ranunculacae), by Ceratophyllum (Callitrichineae), 

 Cabomba (Cabombaceae), Potamogeton (Naiadaceae), Hottonia (Primulaceae) 

 and Myriophyllum (Halorageae). In each of these examples belonging to six 

 different families, the fully submerged leaves are disintegrated in diverse 

 degrees, forming narrow lacineae : while those leaves of them that float on the 

 surface of the water, together with those that are fully subaerial, have 



