62 



ANCIENT PLANTS 



Fig. 37. — Diagram of a Stele with a few Cells 

 of Pith p in the Middle of the Wood. Lettering 

 as in fig. 36 



primitive form which is taken by the vascular tissues, 

 is that of a single central strand, withvthe wood in the 



middle, the bast round 

 it, and a circular endo- 

 dermis enclosing all, as 

 in fig. 36, which shows 

 a diagram of this ar- 

 rangement. Such a 

 mass of wood and bast 

 surrounded by an endo- 

 dermis, is technically 

 known as a stele, a very 

 convenient term which 

 is much used by anato- 

 mists. In its simplest 

 form (as in fig. 36) it 

 is called a protostele, 

 and is to be found in both living and fossil plants. A 

 number of plants which get more complex steles later 

 on, have protosteles in the early stages of their develop'^ 



ment, as in Pteris aurita 

 for example, a species 



^ ,,_ ^ , allied to the bracken 



^ ^^^ ' ' ^^ ^ fern, which has a hollow 



ring stele when mature. 

 The next type of 

 stele is quite similar to 

 the protostele, but with 

 the addition of a few 

 large unspecialized cells 

 in the middle of the 

 wood {p, fig. ^j); these 

 are the commencement 

 of the hollowing process 

 which goes on in the wood, resulting later in the for- 

 mation of a considerable pith, as is seen in fig. 38, where 

 the wood is now a hollow cylinder, as the phloem has 

 been from the first. When this is the case, a second 



Fig. 38. — Diagram showing Extensive Pith/ in 

 the Wood. Lettering as in fig. 36 



