64 



ANCIENT PLANTS 



it is still a convenient term to describe it by. There 

 has been much theoretical discussion about the true 

 meaning of such a "polystelic" stem, which cannot be 

 entered into here; it may be noted, however, that the 

 various strands of the broken ring join up and form a 

 meshwork when we consider the stem as a whole, it is 

 only in a single section that they appear as quite inde- 

 pendent protosteles. Nevertheless, as we generally con- 

 sider the anatomy of stems in terms of single sections, 



and as the descriptive word 

 "polystelic" is a very con- 

 venient and widely under- 

 stood term, it will be used 

 throughout the book when 

 speaking of this type of 

 stem anatomy. 



Such a type as this, 

 shown in fig. 40, is already 

 complex, but it often hap- 

 pens that the steles branch 

 and divide still further, until 

 there is a highly complicated 

 and sometimes bewildering 

 system of vascular strands 

 running through the ground 

 tissue in many directions, but cut off from it by their 

 protective endodermal sheaths. Such complex systems 

 are to be found both in living and fossil plants, more 

 especially in many of the larger ferns (see fig. 88). 



Higher plants in general, however, and in particular 

 flowering plants, do not have a polystelic vascular 

 arrangement, but a specialized type of monostele. 



Referring again to fig. 37 as a starting-point, imagine 

 the pith in the centre to spread in a star-shaped form 

 till the points of the star touched the edges of the ring, 

 and thus to break the wood ring into groups. A stage 

 in this process (which is not yet completed) is shown 

 in fig. 41, while in fig. 42 the wood and bast groups 



Fig. 41. — Monostele in which the Central 

 Pith is Star-shaped, and the Wood break- 

 ing up into Separate Groups 



f. Pith; w, wood; p, phloem; E, endo- 

 dermis; c, cortex. 



