No. 449.] NORTH AMERICAN CONIFERALES. 343 



tendency to the development of ray tracheids as in Thuya and 

 Cupressus. It is also a noteworthy fact that simultaneously 

 with a general thickening of all the cell walls throughout the 

 ray, as in the genus Tsuga, ray tracheids become a constant 

 and prominent structural feature. This relation exists in Pseu- 

 dotsuga, Larix, Picea and Pinus, and it is a remarkable fact that 

 as the type of organization advances, and the structural modifi- 

 cations of the wall become more profound, the tracheids gain 

 steadily in numbers and importance until they finally replace the 

 parenchyma cells more or less completely. Such facts serve to 

 direct attention to the idea that by such progressive alterations 

 the ray cells gradually lose their normal functional powers with 

 respect to the radial distribution of water, and under such cir- 

 cumstances 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 development, 

 and once more resume their appropriate functions. Such struc- 

 tural reductions do in reality occur in these very cases, as shown 

 in Pimis tceda, 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 evidence from other plants in support of a hypothesis 

 of this nature. It is true that in the case of girdled pines the 

 heart wood may resume an activity long since lost, and thus take 

 upon itself once more the function of the sap wood, as also to 

 some extent the function of the bark, but such renewed func- 

 tional 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 stated. Under these circumstances, 

 therefore, it is fair to conclude that such arrested development 

 expresses diversion of energy to the preponderant tracheids. 



The second alternative permits us to consider that in the ordi- 

 nary 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 



