208 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



cells is the bast. For through it the surplus assimilates 

 pass downwards from the leaves to stems, rhizomes, roots, 

 or other storage organs. From the medullary rays the 

 reserves pass to the wood parenchyma cells when these 

 are present. 



The distribution of the parenchymatous cells in the 

 wood shows an orderly arrangement forming vertical 

 plates in connection with the medullary rays, and very 

 regular sheaths surrounding the vessels. The cells abutting 

 on the spring wood are also found to be very rich in starch 

 in many cases. The importance of this arrangement has 

 recently been emphasized by Dixon and Atkins (1915, 1), 

 and in the winter these sheathing cells are differentiated 

 in a striking manner when treated with iodine solutions, 

 owing to their being crowded with starch grains. 



In connection with researches on the ascent of sap, an 

 investigation was carried out by Dixon and Marshall 

 (1915) upon the relative areas occupied in the cross-section 

 of a woody stem by the walls, by the living cells, and by 

 the lumina of the non-living conducting tracts. Since 

 the results have a direct bearing on the present subject, 

 they will be quoted here at some length. 



A QUANTITATIVE STUDY OF THE AREAS OCCUPIED BY 



ELEMENTS OF THE WOOD. 



The method adopted was to obtain photomicrographs 

 of transverse sections of the wood of various trees at such 

 a magnification that, whilst the individual cells were of 

 sufficient size to permit of their lumina being accurately 

 cut out with a sharp-pointed knife, yet a fairly large portion 

 of the section was in the field, so as to minimize irregularities 

 owing to the inclusion or exclusion of medullary ray tissue, 

 which would tend to distort the true proportions. To 

 further avoid this source of error, since the results obtained 



