10 STRUCTURAL BOTANY 



stem. There is no pith whatever ; the whole interior of 

 the cylinder is occupied by solid wood, which consists 

 entirely of tracheides. The development of this central 

 mass of wood is also peculiar, for the first-formed elements 

 or protoxylem-groups lie at the outside of the wood ; in 

 this particular case there are seven such groups, and it is 

 from these points that the development of the xylem starts; 

 so we see that in this stem the wood develops centri- 

 petally, just as it does in the root of other plants. This 

 is a very important difference from flowering plants. 



This centripetal development of the xylem holds good 

 as a general rule l for the stems of the Selaginellas and 

 their allies. 



Surrounding the xylem is a ring of phloem, consisting 

 of parenchyma and sieve-tubes, but with no companion- 

 cells. The sieve-tubes, like those of the Conifers, have 

 their sieve-plates on the lateral walls. The whole stele 

 is bordered by a layer of cells containing starch. Outside 

 this layer is the intercellular space. Each of the cells, 

 which stretch across the space, has a cuticularised band ; 

 these cells represent the endodermis. 



We see, then, that we have a vascular structure in 

 this plant which differs from anything which we have 

 seen before in stems, as shown by (1) the centripetal 

 xylem ; (2) the absence of pith ; (3) the want of separation 

 between the vascular bundles. This type of stele is a 

 very ancient one : many of the plants of the coal period 

 (Lepidodendron, etc.) had a vascular system almost 

 exactly like that of 8. spinosa, though on a much larger 

 scale. This was the case, for example, in stems such as 

 that of which the stump is shown in Part L, Fig. 5. 



1 In the trailing part of the stem of S. spinosa it appears that the 

 protoxylem is central. See also p. 13. 



