592 



BOTANY OF THE LIVING PLANT 



the surface : thus the enlarging thallus is ventilated, and the cell-surface 

 facing the atmosphere tends to be maintained internally (Fig. 448). In 

 Mosses the obconically enlarging protonemal buds become leafy upwards 

 ( Fi gs. 355. 356) : here also the thin leaves as external outgrowths tend to 

 uphold the surface-volume ratio, but by a method the converse of internal 

 ventilation. These simple examples suggest four distinct methods for gaining 



.sec 



■pr 



JV . « m m 



Fig. 447. 



Fritschiella tuberosa. A small mature 

 plant with a single rhizoid. c/=cluster; pr= 

 primary; sec = secondary branch. ( x 350.) 

 (After Iyengar.) 



Fig. 448. 



Riccia trichocarpa. Young sporeling showing 

 spore and germ-tube, k ; rhizoid, r ; and the 

 thallus enlarging upwards with sunken apex, 

 and intercellular spaees beginning to develop. 

 ( x 85.) (After Campbell.) 



that end in obconical sporelings, as their size increases. They are all taken 

 from gametophytes. But both of the alternating phases of the life-cycle 

 are subject to the same demand. For instance, in the sporeling of Poly podium 

 (Fig, 140) the stem is obconical : it increases to about four times its original 

 diameter before its seventh leaf is reached. But here again the surface- 

 vonime ratio tends to be upheld by foliar development, combined with in- 

 ternal ventilation. In point of fact these four methods may be variously 

 combined among themselves in the organisation of vegetation generally. 

 For instance, flattening and internal ventilation take part in the construction 

 of any ordinary leaf-blade; and branching also in the widest sense may be 



