X] 



CONICAL FORM OF YOUNG STEM 



179 



young plant is then not cylindrical, but it is a gradually enlarging cone 

 Consequently problems depending on the proportion of surface to bulk 

 whether of the stem as a whole or of the stele within, will be progressively 

 changing in each successive transverse zone from the juvenile to the adult 

 region. It may be anticipated that at some point of size a critical proportion 

 of surface to bulk will be reached, where the interchanges between stele and 

 cortex will demand some alteration of structure if they are to be satisfactorily 

 carried out. 



Fig. 171. Median longitudinal section 

 through the prothallus and embryo 

 oi Polypodinvivulgare. ( x 6.) Leaves 

 A' 4> titc. ; i? = roots; a/ = apex of 

 stem. The drawing shows the widely 

 expanding conical stem— small at the 

 base, where it is protostelic; larger 

 above, where it is dictyostelic. 



Fig. 172. Deparia Moorei,YiooV. Reconstruc- 

 tion from sections of the vascular system of 

 a plant which had been "starved" by un- 

 successful culture. The normal dictyostelic 

 state is shown below, with wide foliar gaps, 

 and perforations. The stele narrows upwards,' 

 approaching the solenostelic state. (After 

 M^^Lean Thompson.) ( x 12.) 



Conversely, however, an axis or root may diminish progressively in bulk 

 from the base upwards. In a Fern that has been starved by unfavourable 

 culture the size of its stem will be less than it is in the region developed 

 under normal conditions, and the internal tissues follow suit, with simplifi- 

 cation of their structure (Fig. 172). For them the problem, so far as it depends 



