426 THE ANATOMY OF WOODY PLANTS 



pitting referred to above. Not only is the end of the annual 

 increment marked by tangential pitting, but it is also distinguished 

 by the presence of longitudinal tangential storage elements known 

 as wood parenchyma. In the figure these living cells are strictly 

 confined to the face or termination of the annual ring. They are 

 further present as derivatives from tracheary structures, for 

 stages between them and transversely divided tracheids are plainly 

 seen. Both the mode of occurrence and the manner of origin of 

 the first parenchymatous elements in coniferous woods are of 

 equal interest from the climatological and evolutionary standpoints. 

 The position of the tangential storage elements at the end of the 

 annual ring corresponds in an apparently highly significant manner 

 with the appearance of tangential pitting in those woods of later 

 geological time which have developed the phenomenon of annual 

 rings. It has been suggested with a strong degree of probability 

 that tangential pitting in the gymnosperms is an adaptation for 

 supplying abundance of water to the cambium when it renews its 

 activity after the annual period of rest. It is extremely likely 

 that the terminal parenchyma which, so far as our present knowl- 

 edge goes, first made its appearance in the Jurassic is likewise 

 a device favoring the activity of the cambium in a new period 

 of growth. If it is probable that the tangential pitting of the 

 terminal tracheids is for the purpose of supplying the cambium 

 with water, it seems not less likely that terminal parenchyma 

 provides a convenient supply of food for the initial cells of the 

 cambial layer. The conditions present strongly suggest that the 

 first appearance of tangential storage elements was in relation to 

 the nutritive demands of the cambial layer. Whether that con- 

 clusion is accepted or not, there can be no doubt of the correlation 

 of the phenomenon of annual rings and the first appearance of 

 longitudinal parenchyma in the tissues of the wood. This view 

 of the matter is strongly supported by the conditions observed 

 in Paleozoic woods, for these are equally characterized by the 

 absence of annual rings (except of course at the end of the period 

 and in higher latitudes) and of storage elements belonging to the 

 category of wood parenchyma. 



Although at first terminal in position in gymnospermous woods, 

 parenchyma did not long remain restricted to this situation. In 



