104 Agkiculttjeal Experiment Station, Ithaoa, N. T. 



the edge and shows the remarkable enlargement of two of the epidermal 

 cells. Near by can be seen the termination of a vascular bundle. 

 Figure 8 is from the center of the same mesophyll cushion. Passing 

 from each end of the section toward the center an idea can be formed 

 of the progressive elongation of the different cell layers. The tissues at 

 the summit of the cushion are in a state of collapse. This one is 

 remarkable from the fact that the elongation has advanced from the 

 epidermal layer of the under side of the leaf through the parenchyma 

 until the palisade cells of the upper side of the leaf are concerned, and 

 their elongation is in the same direction, *'. e., toward the lower side of 

 the leaf. This results from the fact that the progressive elongation, 

 beginning at the lower side of the leaf and advancing toward the upper, 

 has relieved the pressure from adjacent cells on the lower side, and also 

 that the primary enlargement of the cells of the under side caused the 

 leaf, at this point, to arch downward, wliich in itself would make lower 

 side of the cells the point of the least resistance. Figures 6, 1 and 8 

 are from plants sent by L. R. Jones, horticulturist of the Vermont 

 station. Very rarely, I have noted in a few instances in the forcing- 

 house, cushions are formed uj^on thfe upper side of the leaf. 



The primary effects of this derangement of the tissue, farther than is 

 pointed out above, relates to certain interferences with the life and 

 nutrition of the plant which are manifestly attributable to it. When 

 the cell walls have become so inordinately stretched that they suddenly 

 collapse, as frequently happens, the changes brought about by the 

 escape of water from this and adjacent tissues during the warmer » 

 part of the day may be so profound as to cause the leaf 

 to wilt and die. In other cases the cushions when collapsed soon dry 

 without great loss of water from adjacent tissues, but the injnry is so 

 deep it seriously disturbs the nutrition of that part of the leaf as shown 

 by a yellowing of the iipper surface at this point. Numerous points of 

 attack on the venation of the lower surface of the leaf in such cases 

 gives to the upper surface a spotted appearance, while the dried 

 cushion immediately below presents a tomentose appearance. Althougli 

 in such cases the entire leaf is usually curled upward, the points of 

 injury are more strongly arched, which produces upon the upper sur- 

 face of the leaf a depressed area within the yellow spots. 



The secondary effects of the trouble relate to profound changes 

 which sometimes follow, but which are not so manifestly attributable 

 in all cases, at first examination, to this derangement of the tissues. 

 Frequently, when the leaf does not wilt after the collapse of the 

 cushion, the broken and dying tissues encourage the development of 



