FIBRO VASCULAR TISSUES: PARENCHYMA 53 



quate comprehension of a given problem. The truth of this 

 statement will become more and more obvious as a result of repeated 

 illustrations in the sequel. Much more rarely does the diffuse 

 condition of parenchymatous disposition in the dicotyledons give 

 rise by reduction to a state in which storage elements are to be 

 found only at the end of the annual zones of growth. This situa- 

 tion is exemplified by certain species of the antarctic beech (Notho- 

 fagus), in which, in contrast to all the boreal species of the Fagaceae, 

 the parenchyma is confined to the face of the summer wood and 

 is not distributed throughout the annual ring, as is the rule for 

 the family as a whole. Similar reasoning to that employed in the 

 case of the Salicaceae and certain Magnoliaceae results in the 

 correct conclusion as to the typical and primitive mode of occur- 

 rence of parenchymatous elements. 



It is necessary to emphasize the different interpretations of 

 terminal parenchyma which must be adopted in the case of the 

 conifers and the dicotyledons. In the coniferous series the presence 

 of wood parenchyma on the face of the summer wood is clearly 

 a primitive phenomenon, both because of the comparative and 

 historical data and because of the equally cogent evidence derived 

 from the consideration of the origin of parenchyma cells in this 

 position in the conifers. Clearly, terminal parenchyma in the 

 gymnospermous series is in the act of origination in view of its 

 almost imperceptible transition to septate tracheids. In the 

 dicotyledons, on the other hand, comparative and experimental 

 data alone, in the absence at the present time of any adequate 

 information in regard to the historical evolution of woods of this 

 type, lead to the conclusion that the occurrence of storage elements 

 in the terminal region of the annual rings is rather the result 

 of reduction from a more elaborate and advanced condition (diffuse 

 or vasicentric) than one of primitive simplicity. 



It is obvious that in general among the dicotyledons we need 

 not expect to have as clear evidence in regard to the problem of the 

 derivation of the longitudinal storage elements of the wood as in the 

 case of the gymnosperms, and in particular the conifers, which 

 present themselves to our gaze in so long and continuous a series 

 in geologic time. Evidence for the origin of parenchymatous 



