FIBRO VASCULAR TISSUES: PARENCHYMA 47 



of wood. In certain species of Abies and Tsuga the location of 

 the parenchyma is on the face of the summer wood only, while 

 in others it is no longer strictly confined to this position. In the 

 genus Abies, A. webbiana and A. cephalonica are characterized by 

 longitudinal storage cells no longer wholly terminal in position 

 but scattered throughout the annual growth. The same condition 

 is present in Tsuga mertensiana and in smaller branches of T. 

 canadensis. This type of disposition of parenchyma may con- 

 veniently be designated diffuse to distinguish it from the terminal 

 condition of storage elements in their first appearance among the 

 conifers. It is of interest to note that, so far as the matter has been 

 investigated, both Abies and Tsuga in the root have terminal 

 parenchyma exclusively, no matter what the situation may be in the 

 case of the stem. The diffusion of parenchymatous cells through- 

 out the annual ring in higher forms of seed plants has its significant 

 analogy in the spreading of tangential pitting, at first confined to the 

 later-formed tracheids of the summer wood, to the elements of the 

 rest of the annual ring in higher types. Diffuse wood parenchyma 

 never betrays its origin by normal transitions to septate tracheids. 

 It is only in the case of wounding that its tracheary origin can be 

 distinctly observed. The longitudinal storage cells, to which the 

 name of wood parenchyma is given, always reveal their derivation 

 from the fibrous elements, however, by the fact that they are 

 grouped in series corresponding in length and outline to the pointed 

 tracheids constituting primitively the sole longitudinal elements 

 of the wood. 



In the case of the abietineous genera Abies and Tsuga, cited 

 above, it is quite clear that the storage elements of the woods have 

 in some instances departed from the exclusively terminal position 

 and have become distributed throughout the annual ring. In those 

 conifers belonging to the subtribes known as Taxodineae, Cupres- 

 sineae, and Podocarpineae the parenchyma of the wood is char- 

 acteristically diffuse and no longer, except by the grouping and 

 contour of its elements, gives evidence of tracheary origin. Injuries 

 in most cases reveal conditions of transition from septate tracheids 

 to pointed rows of parenchymatous elements. In the subtribes 

 Araucariineae and Taxineae parenchyma has disappeared from the 



