THE RELATION OF SIZE AND FORM IN PLANTS 593 



involved, giving the condition seen in compound leaves and in leafy shoots 

 at large. When they are associated also with apical growth and lobation 

 of various orders, they lead to that due balance of surface and bulk which 

 has made the realisation of diffuse land-vegetation possible, up to the limit 

 of mechanical resistance (see Chapter X.). 



Statement by Measurement. 



To demonstrate by comparison to uniform scale that there is a 

 real though not an exact relation between Size and Form, it will be 

 convenient to turn to definite internal tracts, and particularly to the 



& 



Fig. 449. 



Outlines of the xylem of Coenopterid steles, all drawn to the same scale, to show their 

 relative sizes ( x 5). 1 = Botryopteris cylindrica. 2=Ankyropteris Grayi. 3=ditto, 

 larger. 4=Asterochlaena laxa. The elaborateness of outline increases with the size. 



conducting tissues of plants of primary construction. In the fossil 

 Fern Botryopteris the small protostele is cylindrical (Fig. 375), and 

 circular in transverse section. But in those of larger size its 

 section is stellate, with projecting flanges, the surfaces between them 



