142 THE EVOLUTION OF PLANTS 



is for the ring of separate strands to be replaced, 

 first, by a continuous zone, and then by a solid 

 cylinder of wood; as we should expect, the series 

 is not perfectly regular, the different types some- 

 times overlapping. Thus normal Osmunda-like 

 stems have been found, not only in the Tertiary, 

 but as far back as Jurassic rocks. In a Jurassic 

 stem, however, from New Zealand, the zone of 

 wood is almost continuous, and in a species from 

 South Africa, of about Wealden age, there is the 

 further peculiarity that wood-vessels occur inside 

 the ring, scattered among the cells of the pith. 

 All these forms are referred to the genus Os- 

 mundites. 



Some extremely interesting stems are re- 

 corded from the Upper Permian of Russia, 

 showing what the Osmundaceous structure was 

 like at the close of the Palaeozoic period. It is 

 rare to find fossil plants at this horizon, and in 

 this case the result justifies our curiosity as to the 

 Flora of these rocks. In one species, Zalesskya 

 gracilis, the wood forms a perfectly continuous, 

 very broad zone, with no sign of division into 

 separate strands. A very curious point is that 

 while the outer part of the zone has the ordinary 

 wood-structure, the inner consists of shorter and 

 wider vessels, such as we find, in recent plants, 

 used for storing water, rather than for conducting 

 it up the stem. In this form there was still a 



