New York Agricultural Experiment Station. 351 



To the naked eye a piece of cucumber leaf is structureless; 

 but if a very thin cross-section is cut and placed under a com- 

 pound microscope which magnifies about 390 diameters, it is 

 found to be made up of numerous compartments or cells, some 

 of which contain many green bodies, the chlorophyll grains. Fig. 

 ], Plate XI, is a drawing of a small portion of such a cross- 

 section. Above and below there is a layer of colorless cells (a, d) 

 called the epidermis {a is on the upper face of the leaf, d on the 

 lower). The epidermis is an impervious protective covering for 

 the leaf. Between a and« we find cells of various shapes. Near 

 the upper surface of the leaf they are much elongated and are 

 called palisade cells (6, 6). Toward the lower surface they are 

 more nearly spherical. In each of these cells there are several 

 small green bodies (c) the chlorophyll grains which give the green 

 color to the leaves. Fig. 2 of the same plate is a drawing of a 

 portion of the epidermis or skin, peeled from the lower surface 

 of the leaf. The elliptical objects (s), are stomata (sing, stoma). 

 Between the two cells composing a stoma there is a narrow slit 

 (r) which opens into an intercellular passage on the interior of 

 the leaf. At m in Fig. 1, there is shown a cross-section of the 

 two cells of a stoma, just beneath the intercellular passage (i). 

 On the under surface of the leaf the number of stomata to the 

 square inch is more than 400,000, while on the upper surface 

 there are about 165,000 per square inch. The epidermis of the 

 upper surface of the leaf resembles closely that of the lower sur- 

 face, except that there are fewer stomata. On both surfaces of 

 the leaf there are hairs which can be seen with the naked eye. 

 These hairs are of two kinds: (1) long, tapering hairs like the 

 one shown in Fig. 2, and (2) short hairs with large, swollen 

 tips, called glandular hairs. 



NATURE OF THE DOWNY MILDEW. 



The symptoms of this disease have been given on a previous 

 page. The naked eye can detect nothing about the diseased 

 leaves which could cause the yellow spots and consequently they 

 are a puzzle to farmers. As in the case of many other plant 



