iqi-] STOBER— WINTER AND SUMMER LEAVES 93 



From the tabulated results of observations made on these 

 species of plants and a careful study of a number of other species, 

 the following conclusions can be formulated. (1) Epidermal hairs 

 are most abundant on the upper stem leaves, and decrease, as a 

 rule, to the lowest stem leaves, and from the upper to the lowest 

 rosette leaves. On the basal leaves of both stem and rosette are 

 found the smallest number of hairs. (2) Hairs are also more 

 abundant on the lower than on the upper surface of the leaf, 

 usually being most abundant on the ribs, veins, and margin of the 

 leaf. (3) Hairs are most abundant toward the base of leaves, 

 although in basal stem and rosette leaves the reverse is usually the 

 case. (4) Young leaves are more hairy than older ones. This 

 may be due partly to the fact that in young immature leaves the 

 epidermal cells have not yet reached their mature size and there- 

 fore the hairs will of necessity be more crowded than in a mature 

 leaf. This diminished hairiness in older leaves also may be due in 

 part to the fact that hairs in the course of time may break off, or 

 for some reason or other drop off, and thus reduce the number per 

 unit area of surface. (5) Exposure to sun, wind, and other desic- 

 cating influences tends to increase the hairiness in the upper stem 

 leaves. Transpiration, wind, moisture, and character of soil are 

 undoubtedly potent factors in determining hair production, but 

 that these are not the only factors is clearly shown by the fact 

 that young leaves just emerging from buds, and therefore most 

 protected, are usually most hairy, sometimes even tomentose. 



As stated before, some leaves are most hairy toward the base, 

 where the leaf is most protected from those influences that would 

 ordinarily tend to produce hairiness. It is difficult also to see why 

 Verbascum Tkapsus and V. Blattaria should grow side by side, the 

 one glabrous and the other extremely hairy. So far as hairiness 

 is concerned, it would seem that the former is a congenital xerophyte 

 while the latter is a congenital mesophyte. 



It is difficult also to see that hairiness is beneficial to plants, 

 and that these epidermal outgrowths protect the plant against 

 excessive transpiration, against the ravages of animals and para- 

 sites of various kinds, against excessive sunlight, etc., when Ver- 

 bascum Blattaria, entirely devoid of hairs and with only a slightly 



