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cellular spaces. In t lie spongy parenchyma numerous thick-walled 

 sclerenchyma fibres are found, which are generally ramified and 

 often have an enormons length. 



A section through a leaf-tip on which yellow vesicles are found, 

 shows that these vesicles are caused by hypertrophy of cells of the 

 spongy parenchyma which have there entirely lost their tubular shape 

 and among which specimens are found, measuring 91 by 109 (i, 

 100 by 73 i>, 100 by 113 y. etc. Among the cells, constituting (he 

 vesicle, some are always found which in unstained preparations are 

 conspicuous by their wall being more or less swollen and brown. 

 In preparations, treated with acid fuchsin and methyl green, the wall 

 of these cells is blue, those of the other cells red. Applying the 

 usual reactions we find that these walls have become suberized. Also 

 of the palissade parenchyma some cells may have become larger, 

 but always in a small degree. 



While in the vesicles themselves the process goes a little further 

 still on account of partition walls forming in some of the enlarged 

 cells, whose walls have not become suberized, also the region, 

 surrounding' the vesicles, evidently answering to a stimulus, proceeding 

 from them, begins to undergo similar changes. Macroscopically we 

 detect this by the more or less yellow tint, assumed by the vicinity 

 of the vesicles. A microscopical examination of the section shows 

 that now also outside the vesicles the cells of the spongy parenchyma 

 are hypertrophical. As with the formation of the vesicles the chlo- 

 rophyl is disorganised in the hypertrophical cells. 



At this stage no function of importance may be ascribed yet to 

 the cells belonging to the palissade parenchyma. 



In most preparations now already the peculiar behaviour is noticed 

 of those spongy parenchyma cells which border immediately on the 

 palissade parenchyma. While the other cells of the spongy parenchyma 

 swell as evenly as possible in all directions, those which lie imme- 

 diately below the palissade parenchyma become enlarged especially 

 in a radial direction, thus making the impression of a second layer 

 of palissade cells. Since also in later stages they will repeatedly 

 draw our attention, I shall in what follows call these cells subpa- 

 lissade cells, instead of using the cumbrous longer definition. 



This extension, especially in a radial direction, of the subpalissade 

 cells, is illustrated by the following two tables. 



A Subpaliss. cells of : 

 normal leaf-tip 



B Subpaliss. cells of 

 yellow leaf- tip 



