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CHAPTER VII 



THE ORIGIN OF THE STELE IN THE 

 EARLIER CORMOPHYTA 



The discovery, or rather the confirmation of the discovery, of 

 an extremely primitive stele in species of Psilophyton arid also 

 of Arthrostigma has naturally a very important bearing on our 

 notions of the origin of the stele in Pteridophyta. 



Among the oldest fossil plants, of far greater antiquity than 

 the very ancient land plants under consideration here, there are 

 many multi-cellular types without any trace of a specialised 

 conducting tissue. These fossils commonly occur in marine rocks 

 and they so closely resemble types of living Algae in their 

 general structure that we are justified in regarding them as 

 marine Algae. 



Somewhat later in. point of time, in the marine Silurian rocks 

 and extending into the early Devonian, we find another type of 

 anatomical habit in which the whole thallus is tubular. This 

 type is represented in Nematophycus where the tubes are some- 

 times of two different calibres, the smaller forming a dense 

 packing between the larger tubes. This is however a feature also 

 common to some living Algae and needs no further remark here. 



Among living Algae no more advanced type of water con- 

 ducting tissue is met with, though some Algae have evolved 

 what we regard as a quite typical phloem. The fact that nearly 

 all the higher living Algae, other than the symbiotic types 

 (Lichens) are hydrophytes and not terrestrial plants, furnishes 

 a ready explanation of the absence of a lignified conducting 

 tissue. There is no need of such a tissue. 



When however we come to Devonian land plants we find in 

 ■Psilophyton and Arthrostigma and other types an extremely 

 early stage in the evolution of the stele. What we have is a 

 single protoxylem group alone, formed by the simultaneous 

 modification of a set of procambial elements. That modification 



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