156 



palissade parenchyma become enlarged especially in a 

 radial direction, thus making the impression of a second 

 layer of palissade cells (PI. II, fig. 11). Since also in later 

 stages they will repeatedly draw our attention, I shall in 

 what follows call thèse cells suhpalissade cells instead of 

 using the cumbrous longer définition, 



This extension, especially in a radial direction, of the 

 subpalissade cells, is illustrated by the following two 

 tables. 



A Subpaliss. cells of 

 a normal leaf-tip 



B SubpaUss. cells of 

 a yellow leaf-tip 



42 



In a leaf-tip which macroscopically is distinguished, 

 besides by the yellow colour, by a distinct thickening (PI. 

 III, fig. 13), the mtercellular cavities of the spongy paren- 

 chyma are foimd to hâve entirely disappeared, excepting 

 a small corner hère and there. The mutual pressure which 

 the cells consequently begin to exert on each other, causes 

 them to assume a more polygonal shape. The cells, bor- 

 dering on the subpalissade cells and often also the rows, 

 turned towards the lower epiderm, show a tendency to 

 stretch themselves in a direction perpendicular to the sur- 

 face of the leaf. In many spongy parenchyma cells par- 

 tition walls hâve formed. 



The part, played by the subpalissade cells in the process 

 of thickening, is generally a very important one. So I 

 found in a leaf-tip in a place, where it was 332 m- thick 

 (the section of a normal tip is on the average 170 /n), cell 

 rows, formed of subpalissade cells, partitioned by two or 

 three walls and measuring 90, 110, 115 and 127 f^ in 



