40 THE ANATOMY OF WOODY PLANTS 



which throws light on this important subject. In the stem of the 

 remaining vascular plants the problem has likewise become obscured 

 by lapse of time and the blotting out of the original and primitive 

 conditions. There seems to be no evidence in regard to the origin 

 of parenchymatous elements in primary wood other than that 

 in the stem of the lepidodendrids, described above. The modern 

 descendants of the group do not furnish elucidation of the origin 

 of parenchymatous elements, and the same statement appears to 

 hold in the case of other living and extinct representatives of the 

 Pteridophyta. 



The secondary wood of the older and Paleozoic groups of plants 

 was, so far as our present knowledge goes, entirely without paren- 

 chymatous or storage elements (with the exception of the so-called 

 medullary rays to be dealt with in the next chapter). This is 

 true not only of the arboreal cryptogams of the ancient period, the 

 lepidodendrids, the sigillarias, the sphenophyllums, and calamites, 

 but also of those lower and more primitive gymnosperms included 

 under the general headings of Cycadonlicales (Pteridospermae) 

 and Cordaitales. Not only are the Paleozoic vascular plants not 

 known to manifest the presence of true xyliary parenchyma, but 

 the same statement appears to hold in regard to the vascular plants 

 of the early part of the Mesozoic that is, the Trias. Parenchyma- 

 tous elements have not yet been seen in any American wood of the 

 earlier Mesozoic. It is only in the Jurassic period that paren- 

 chymatous cells clearly manifest themselves as a feature of the 

 organization of the secondary wood, and at this time also the 

 annual ring as a feature of organization of the woody cylinder 

 first becomes well marked. There is, in fact, a distinct correlation 

 between the appearance of true parenchymatous storage elements 

 and the phenomenon of annual rings. 



Before referring to true parenchymatous structures in secondary 

 wood it will be well to make mention of structures which may 

 readily be mistaken for these. In certain plants even of the 

 Paleozoic age the rays have attached to their margins more or 

 less elongated cells which are often of such length that they form 

 longitudinal commissures between adjacent rays. A situation of 

 this kind is found, for example, in the calamites and sphenophyllums 



