MEDULLARY RAYS 91 







and under such circumstances it is imperatively demanded that 

 this deficiency should be met through some other structures. 

 Under these circumstances two alternatives are possible : first, 

 that the thick-walled and useless cells should return to their 

 primitive condition in opposition to the general course of devel- 

 opment, and once more resume their appropriate functions. Such 

 structural reductions do in reality occur in these very cases, as 

 shown in Pinus taeda, etc., but it is to be observed that they are of 

 the nature of a growth which has been arrested at such an early 

 stage as to be devoid of many of the normal structural features. 

 Furthermore it would be difficult, if not impossible, to obtain evi- 

 dence from other plants in support of a hypothesis of this nature. 

 It is true that in the case of girdled pines the heartwood may 

 resume an activity long since lost, and thus take upon itself once 

 more the function of the sapwood, and also to some extent 

 the function of the bark ; but such renewed functional power 

 does not in any way involve structural modifications of existing 

 elements, and cases of this sort cannot be cited in support of the 

 hypothesis. It is therefore fair to conclude that such structural 

 reduction and restoration of functional activity are accompanied 

 by a partial diversion of energy to the preponderant tracheids. 



The second alternative permits us to consider that in the 

 ordinary course of development the ray cells gradually lose their 

 functional activity as a result of extreme structural modification, 

 and that this loss of power cannot be restored, even though the 

 wall may return to a primitive condition of structure through 

 various phases of atrophy. In accordance with this idea the 

 tracheid would be introduced as the most natural because the 

 original medium for such activities as are centered in the ray, 

 and it would therefore acquire additional importance both nu- 

 merically and functionally in direct proportion to the loss of 

 power experienced by the parenchyma cells. This appears to be 

 a reasonable interpretation, and in the light of observed facts 

 it would seem to be the correct one. 



A structural feature of great importance in the ray tracheid 

 appears in certain inequalities of the upper and lower walls, which 



