THE RELATION OF SIZE AND FORM IN PLANTS 601 



The Size Problem in the Bryophytes. 

 Those surfaces of a plant which are in contact with a surrounding 

 medium, such as air or water, are styled presentation-surfaces, whether 

 these be external or internal. For instance, in order to uphold gaseous 

 interchange in subaerial plants it is essential to maintain a due pro- 

 portion of surface to bulk ; and the like is needed also in partially or 

 completely submerged plants. To this end Mosses and Liverworts 

 have adjusted their form and structure up to a point, though without 

 realising their opportunities to the full. Accordingly they afford 

 interesting comparisons with other plants. What is commonly lacking 

 in their organographic make-up is the combination of sufficiently 

 elaborate external contour, with or without internal ventilation. For 

 instance the gametophyte of the Jungermanniales is commonly leafy, 

 though without internal ventilation (Fig. 369) : on the other hand, the 

 Marchantiales have an internal type of ventilation of the branched 

 and fleshy thallus (Fig. 368) : meanwhile the unbranched sporo- 

 gonia of both are without ventilation. In the Mosses the plant is 

 leafy, and often profusely branched : but even the largest of them 

 lack the ventilated structure of Vascular Plants. Their sporogonia, 

 however, have internal ventilation, with localised stomata (Fig. 364) : 

 but they are unbranched and leafless. Neitlier phase of the life-cycle 

 in these plants has secured full efficiency, by combining well-developed 

 ventilation with branching of a leafy shoot, as Vascular Plants have 

 done. In this we may see a reason for their limited stature. As a Class 

 they are structurally doomed to be dwarfs. 



Combination of Organographic Factors. 

 That combination which is wanting in the Bryophytes is, however, 

 present, together with other helpful features, in most Vascular Plants. 

 The success of their vegetative system in developing to large size 

 is chiefly based on such features as, (i) continued apical growth ; 

 (ii) branching of various orders ; (iii) internal ventilation ; (iv) a 

 plastic primary conducting system ; and (v) secondary cambial 

 increase in many, but not in all. Collectively, and in various com- 

 bination, such features as these lead to the diffuse form and elaborate 

 structure seen in Land Vegetation. On the other hand, by comparing 

 the striking results that have followed in Vascular Plants with the 

 limited success of the Bryophytes, we may measure the importance 

 of those combinations which the Mosses lack. But, as a consequence 

 of the superiority of the Higher Plants based on such advantages as 



