igi-] STOBER- WINTER AND SUMMER LEAVES 107 



80 per cent of the types studied, is greater in rosette than in stem 

 leaves. This is correlated perhaps with the greater thickness of 

 the chlorenchyma in the former. The thickness of palisade cells, 

 in at least 90 per cent of all cases, is also greater in rosstte than in 

 stem leaves. 



5. The average size of spongy parenchyma cells is also greater 

 in rosette than in stem leaves. This is true of about 90 per cent 

 of all plants studied. 



6. Sclerenchyma tissue seems to be about equally well developed 

 in ribs and veins of both kinds of leaves. The conductive tubes 

 in veins of approximately equal size have a slightly larger lumen 

 and a wall slightly thicker in rosette than in stem leaves. The 

 conductive system of rosette leaves is better developed in rosette 

 than in stem leaves, although this rule is not without exceptions. 



On the whole, therefore, it may be said that, so far as the 

 structure of chlorenchyma is concerned, stem leaves are more 

 xerophytic in structure than rosette leaves, although the latter 

 appear to be more xerophytic so far as the greater thickness of 

 epidermal wall and cuticle are concerned. In some instances the 

 xerophytic character of stem leaves, as compared with the rosette 

 leaves of the same plant, is so pronounced as to be easily detected 

 with the naked eye. 



Conclusions 



1. Hairs are most abundant in the upper stem leaves and 

 decrease to the basal leaves; they are also most abundant in 

 the upper rosette leaves and decrease to the basal leaves. In 

 general, however, the stem leaves are more hairy than the rosette 

 leaves. 



2. Stomata are usually smaller, more nearly round, and more 

 abundant, per unit area, on stem than on rosette leaves. 



3. As a rule, the epidermal cells of rosette leaves are larger than 

 those of stem leaves and have more sinuous lateral walls. The 

 shape of the cells is usually correlated with the shape of the leaf. 



4. The blade of rosette leaves is thicker than that of stem 

 leaves, chiefly owing to a greater development of spongy paren- 

 chyma. This is not true, however, of stem leaves that are long, 



