62 



ANCIENT PLANTS 



W 



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

 of Pith / 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, with the 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- 



E I m ''''- '''/'\ -ft ) $ rangement. Such a 



P_| BllXlipPfe ~T^ 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 



Fig. 38. Diagram showing Extensive Pith /in WOOU \p, tig. 37); tnCSC 



the wood. Lettering as in fig. 3 6 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 



