

io6 



BOTANICAL GAZETTE 



[FEBRUARY 



upper and lower epidermis frequently have an outer wall of approxi- 

 mately equal thickness, the upper epidermis has a thicker wall than 

 the lower. In 93 per cent of the cases the outer epidermal wall 

 and cuticle of stem leaves were found to be thicker in stem than in 

 rosette leaves. The thickest epidermal walls are usually found in 

 the outer two-thirds of upper stem leaves. On the other hand, 

 rosette leaves have epidermal cells with the largest vertical diameter 

 and contain a maximum of air spaces. 



Lepidium virginicum and Capsella Bursa- pastor is produce both 

 summer and winter rosettes. When these rosettes are compared, 

 it is found that summer rosettes have slightly thicker leaves 

 (thinner in Capsella), a thicker cuticle, and a thicker outer epider- 

 mal wall. The palisade parenchyma also is better developed. 

 There are frequently more layers, and the cells are longer and 

 narrower. These differences are most pronounced in Lepidium. 



Summary on chlorenchyma. — Typical xerophytic leaves 

 have a relatively compact and well developed palisade tissue; also 

 a relatively small amount of spongy parenchyma with small air 

 spaces. The mechanical tissue is usually also better developed 

 than in mesophytic and shade leaves. Since rosette leaves are 

 usually broad, close to the ground, frequently more or less shaded, 

 and therefore in most respects better protected than stem leaves, 

 it should not be surprising if the former were found to be more 

 mesophytic than the latter. That this seems to be true, at least 

 of the forms studied, is shown by the following deductions. 



1. Rosette leaves, as a rule, have a greater amount of chloren- 

 chyma than stem leaves. This is true of at least 80 per cent of all 

 plants studied. 



2. Rosette leaves have a greater amount of spongy parenchyma 

 than stem leaves, although the percentage of the chlorenchyma 

 is slightly greater in the latter than in the former. 



3. The percentage of air spaces in both palisade and spongy 

 parenchyma is also greater in rosette than in stem leaves. This 

 is true of about 86 per cent of all plants studied. In a considerable 

 number of instances, however, the differences are slight. 



4. The number of palisade layers is much the same in both 

 kinds of leaves, but the average length of palisade cells, in at least 



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