212 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



and penetrate into the medullary rays at different levels 

 for transmission to the wood, which is the principal store- 

 house. After this they travel through the vessels leading 

 to the regions in which growth is taking place. 



" However, if the role of the medullary rays as a tissue for 

 the transference of reserves has been sufficiently insisted 

 upon, their great importance as the storehouse of the stem 

 has not been adequately emphasized. This function may 

 be easily understood when one remembers that in certain 

 trees, as already mentioned, the rays represent a very large 

 proportion of the total mass (one-fifth or even one-fourth), 

 and that all this tissue, at certain times of the year, is 

 closely packed with foodstuffs (starch). It is true that 

 the wood parenchyma and wood fibres (Salix, Acer, etc.) 

 also at times contain for their part large quantities of these 

 substances, but for the major portion of the year the 

 quantity contained in the rays is much superior. This role 

 of the medullary rays becomes even more important in 

 trees in which the wood parenchyma is poorly repre- 

 sented. 



" As for the division of physiological labour between the 

 two elements composing the medullary rays, we can now 

 be more explicit. In fact, we see that, when starch 

 reappears in spring, it fills the two kinds of ray cells, 

 with the exception sometimes of those of the upright 

 approximately cubical cells abutting on the vessels. When 

 the starch is dissolved, it ordinarily disappears first of all 

 in the radially elongated cells. The order in which this 

 solution is effected becomes more marked according as the 

 morphological differentiation of the two types of element 

 becomes more complete (Salix, Celsius, Liquidambar, 

 Viburnum, etc . ) . The order j ust described is the usual one, 

 although it so happens that certain trees show another 

 order for the solution of the starch, in spite of the pro* 



