i 9 i 7 ] STOBER— WINTER AND SUMMER LEAVES 101 



studied. The thickness of the leaf, the thickness of the palisade 

 parenchyma, the thickness of the spongy parenchyma, the number 

 of the palisade layers of cells, and the average size of the cells of 

 each layer, together with the size, shape, and arrangement of the 

 cells of the spongy parenchyma, were the leading points of obser- 

 vation in this comparative study. All measurements are expressed 

 in microns, and were made approximately 800 /x from the midrib 

 of the leaf. For want of space the tabulated results cannot be 

 given; a general summary in each case must suffice. 



Lepidium virginicum. — In the upper stem leaves the palisade 

 parenchyma is almost equally developed in both the upper and 

 lower side of the leaf. This may be due to the fact that the leaves 

 stand at a very acute angle to the stem and are almost equally 

 illuminated. In the middle stem leaves a lower palisade tissue is 

 found only in the apical region of the leaf. No lower palisade 

 layers are found in the lower stem leaves or in any of the rosette 

 leaves. The palisade layers of the upper stem leaves are quite 

 compact. The cells reach a maximum length in the middle stem 

 leaves. In the basal stem leaves the cells become larger and more 

 rounded, the layers are less closely packed and less definitely organ- 

 ized. The palisade cells of the rosette leaves are larger, having 

 a decidedly greater diameter, and on the whole are less compactly 

 arranged than in stem leaves. The upper and middle stem leaves 

 have the thickest outer epidermal wall and cuticle. This is also 

 true of the upper rosette leaves when compared with the lower. 

 The difference, however, between the thickness of the epidermal 

 wall and cuticle of the apical and basal leaves is much greater in 

 rosette than in stem leaves. 



Capsella Bursa-pastoris— The outer epidermal wall of stem 

 leaves is thicker in the upper than in the basal leaves, attaining a 

 maximum in the middle leaves. The cuticle is proportionally 

 thickest in the upper leaves and thinnest in the proximal part of the 

 basal leaves. Similar conditions obtain in the rosette leaves, 

 except that the contrast between apical and basal cells is less pro- 

 nounced. Palisade tissue is best developed in both upper stem 

 and upper rosette leaves. Palisade cells are slightly longer and 

 decidedly thicker in rosette than in stem leaves. The cells of the 



