602 BOTANY OF THE LIVING PLANT 



these, the Bryophytes do not offer the best field for study of the Size 

 and Form Relation. There are in fact three grades of vegetation, 

 differing in the degree of their adjustment to the progressive demands 

 of increasing Size, though they are not sharply marked off from one 

 another : 



I. Rudimentary organisms, of submerged or semi-aquatic habit, 

 which are for the most part built up on the cylindrical filament, 

 such as Algae and Fungi. In these the maintenance of the surface- 

 volume ratio does not present serious difficulty. 



II. Ordinary Land Plants, which have met in full the demands of 

 increasing size by the aid of structural changes recognised as second- 

 ary, such as internal ventilation and cambial increase. 



III. Primitive Land Plants known as the Archegoniatae, which 

 occupy a middle position, their organisation in many instances being 

 based on primary structure only. 



The study of Morphology in Plants seems to have progressed as 

 though there were no Size Problem. One reason for this has probably 

 been that the simplest plants of aquatic habit do not present the 

 question in any acute form : while the higher plants of land habit 

 have solved their problem by adaptive adjustment. But the earliest 

 denizens of the Land, that is the Archegoniatae, have either carried 

 out such adjustments imperfectly, or not at all. It is through them 

 that the Problem of Size and of its consequences may be most readily 

 grasped : and in none of them is the evidence so clear as it is in the 

 Mosses and Ferns. 



Mechanical Limitations. 

 The foregoing pages have touched upon those various devices by 

 which plants meet more or less fully the physiological demands 

 that follow inevitably on increase in Size. But the direct demand 

 for mechanical stability stands at the back of all progressive organisation. 

 As the size increases the strength of a structure increases as the 

 square of the linear dimensions, but the weight or mass as the 

 cube, provided the form and material remain the same. We have 

 seen in Chapter X. how the simplest steps to secure stability are based 

 on the turgidity of the encysted cell, which suffices for small organisms, 

 chiefly aquatic. In land plants of larger size the necessary resistance 

 may be secured by the help of cell-partitioning and of specialised 

 sclerotic tissues, still of primary origin. A climax of such primary 

 development has been reached with amazing success by Tree Ferns, 

 Bamboos, Screw Pines, and Palms. But the most successful develop- 



