FIBROVASCULAR TISSUES: PARENCHYMA 45 



clear in a former chapter that in the case of the Paleozoic 

 gymnosperms there are at the same time no annual rings (typically 

 at leas.t) and the pitting of the tracheary elements of the wood 

 is entirely radial in its disposition. In the gymnosperms of the 

 Mesozoic age both annual rings and tracheids tangentially pitted 

 in the terminal region of the yearly zones of growth become the 

 rule. It is highly significant in this connection that the first 

 appearance of true longitudinal storage parenchyma is from the 

 Jurassic (Middle Mesozoic) onward, and that the storage cells take 

 their origin in the terminal region of the annual rings and clearly 

 show from their mode of development that they are the derivatives 

 of tracheary elements, since all stages of transition between septate 

 tracheids proper and files of parenchyma are found. It has hjeen 

 suggested by Strasburger that the tangential pitting of the sum- 

 mer tracheids is for the purpose of easily and rapidly supplying 

 water to the cambium in the next opening period of growth. 

 Such a hypothesis would accord well with the very definite correla- 

 tion between tracheids of this type and the phenomenon of annual 

 rings. If the condition of annual growth be accepted as the prob- 

 able elucidation of the tangential pitting of tracheids in more 

 modern gymnosperms, it is not difficult to put forward a similar 

 claim for the terminal and tangential parenchyma which probably 

 represents the primitive disposition of parenchymatous elements in 

 gymnospermous woods. If the tangential pitting is for the purpose 

 of supplying the awakening cambium with water, there can be little 

 doubt that the later-appearing and correlated feature of terminal 

 longitudinal storage cells is significant in connection with the need 

 of the reviving initial cells of the zone of growth for readily avail- 

 able food. 



In Pinus, as indicated above, there is no true wood parenchyma. 

 In the allied genera, Picea, Pseudotsuga, and Larix, it has become 

 clearly and unmistakably established and occurs everywhere in the 

 wood of the root, even when it is absent or degenerate in the stem. 

 As will be shown later, the root is the most conservative organ 

 of plants. In the case of higher members of the pine family (Abie- 

 tineae), Cedrus, Pseudolarix, Abies, and Tsuga, the parenchymatous 

 elements are still found typically at the end of the annual ring, but 

 they no longer normally show evidence of derivation from tracheids, 



