192 TRANSLOCATION IN PLANTS 



direction for sucrose and in the reverse direction for hexoses 

 may not actually obtain in the conducting elements 

 themselves. 



The studies of Mason and Maskell (19286, 1934), 

 Maskell and Mason (1930a, 1930c), and Phillis and Mason 

 (1933) give a much clearer picture as to the form in which 

 carbohydrate is transported and the concentration gra- 

 dients in the phloem. Other investigators had determined 

 concentrations of sugars by analyzing entire veins or 

 petioles, which therefore included xylem and parenchyma 

 tissues with the phloem. Mason and Maskell not merely 

 separated bark from wood but subdivided the bark into 

 outer, middle, and inner portions. 



The distribution of the phloem is not uniform through- 

 out these regions of the cotton bark but increases markedly 

 in proportion to the general parenchyma as one approaches 

 the cambium (see Fig. 11). The phloem with its attendant 

 sclerenchyma forms wedge-shaped groups with the base 

 of the wedge facing the cambium, while between these 

 groups are wedge-shaped masses of general parenchyma 

 with the base of the wedge facing the outside. The inner 

 layers of bark, therefore, contain a relatively high content 

 of phloem and low content of cortical parenchyma, while 

 outer layers contain high cortical parenchyma and low 

 phloem. The outer phloem is also older and much of it 

 may therefore be nonfunctional. 



Data showing the radial distribution in the bark of 

 sucrose and reducing sugars as well as of several nitrogen 

 fractions are given in Table 21 and Fig. 11. These clearly 

 indicate that sucrose is the predominant sugar in the inner 

 tissues which are chiefly phloem proper, and reducing 

 sugar is predominant in the outer cortical parenchyma. 

 The phloem region is also richer in crystalloid nitrogen, the 

 significance of which is briefly discussed at the end of this 

 section. Not only does the high sucrose content of the 

 phloem (which they call ''sieve tubes") indicate that this 

 is the transport sugar, but this is supported by their 

 finding that any marked changes in sugar content of the 



