THE RELATION OF SIZE AND FORM IN PLANTS 597 



the supply of nutrients required in the growing region. All this is, however, 

 achieved in Palms in the absence of secondary accretion. As such primary 

 development is more readily apparent in Ferns than in Monocotyledons, the 

 former are selected here to illustrate steps of stelar expansion in relation 

 to the enlargement of the terminal bud. 



The Expanding Stele in Ferns. 



The obconical enlargement habitual in the stems of sporeling Ferns is clearly 

 seen in Polypodium vulgare (p. 208, Fig. 140). Not only does the axis enlarge 

 upwards, but the stele likewise expands. Comparison shows the steps usual in 

 them to be through medullation to solenostely (Fig. 452), and by overlapping 

 of closely disposed leaf-gaps to dictyostely (Fig. 376A). This has been described 



Fig. 453. 



Series of transverse sections of the stem of Pteris (Litobrochia) podophylla, all drawn 

 to the same scale, so as to show the great increase in size, and the progressive complexity 

 of the conducting tracts (shaded) as the stem expands conically upwards : also the 

 successively enlarging leaf-bases attached laterally in each. ( x 4.) 



briefly on pp. 485-7. It is in fact general in Leptosporangiate Ferns. For full 

 details it must suffice to refer to Ferns, Camb. Press, Vol. I., Chapters VII., 

 VIII. : Primitive Land Plants, Chapter XVII. and Size and Form, Chapter 

 VIII. : where references are given to the extensive literature on the subject. 

 This short precis of facts, together with the illustrations which follow, will 

 serve as a basis for discussion of the relation of obconical growth to the 

 expanding primary stele of the adult Fern. 



In the Ferns these elaborations all start from the simple protostele, which 

 expands upwards with the enlarging shoot. A central pith is then formed, at 



