(Edema of the Tomato. 103 



but the crowding one upon another has displaced many others. A 

 study of the series of sections from such a cushion, as well as the study 

 of one in an early stage of development, shows that the epidermal cells 

 nearly always first partake of the radial elongation. These are followed 

 by the sub-epidermal layer and so on. 



Serial sections through the cushions on the stem were also made. 

 Three from a series through quite a young cushion on the stem are 

 reproduced by camera lucida drawings in figures 9, 10 and 11. 



Figure 9 represents a section taken from the edge of the cushion and 

 shows the early stage in the elongation of a few epidermal cells. In 

 this section is also shown a peculiarity in the early elongation of some 

 of the more deeply seated cells. A few of the cells of the collenchyma 

 have elongated radially before those lying immediately outside of them 

 have changed their form. This rarely occurs, and by the time the sec- 

 tions approach the center of the cushion the successional changes of the 

 layers of cells from the outside exists in the usual manner It is to be 

 noted in connection with the elongation of the cells of the collenchyma, 

 that while such thick-angled tissue offers support to the stem, little more 

 resistance is offered to the radial elongation of the cells by this tissue 

 than by parenchyma. The lateral walls being thin permit the cell 

 membrane to be stretched. Figure 11 is one of the series from the 

 center of this very young cushion. Figure 1-2 is from an older cushion 

 on a much older part of the stem of the same variety. Here many of 

 the epidermal cells have sloughed off and others are nearly free. The 

 hypodermat layer, chlorophyll bearing layer, is remarkably elongated, 

 ' while the first layer of the collenchyma is also changed. In the older 

 cushions the radial elongation effects also deeper layers of the collen- 

 chyma, sometimes two or three layers. Usually by the time such 

 changes occur to any marked degree in the deeper layers, the sub-epi- 

 dermal portion of the cushion is in a state of collapse. This effect is 

 partially shown in figures 13 and 15. 



On the leaves of some plants there frequently is very little evidence 

 to the unaided eye of the presence of these cushions. But here and 

 there, upon close examination, can be seen very minute elevations. 

 Magnified with a pocket lens they present the appearance shown in 

 figure 6. They appear to be located entirely in the mesophyll of the 

 leaf, or more properly speaking, at or near the terminations of some of 

 the veinlets. This probably accounts for their small size. Being 

 located in such delicate tissue they rapidly collapse and disappear from 

 view. 



Figures 1 and 8 represent sections selected from serial sections 

 through one of these small mesophyll cushions. Figure 7 is taken from 



