9 2 



BOTANICAL GAZETTE [February 



With such a tendency to variation in plants, few measurements 

 and counts can be regarded as absolutely fixed, but the final results 

 in any case do not materially affect the principles involved. 



Epidermal hairs 

 There is considerable variation in the kind, number, size, and 

 distribution of epidermal hairs, not only in different plants of the 

 same species, but also on different leaves of the same plant, or even 

 on different parts of the same leaf. Some plants, such as Oenothera 

 biennis, vary greatly when grown under different physical condi- 

 tions. In a low, moist, and comparatively shady habitat the leaves 

 of Oenothera are thin, and the hairs rather weak and comparatively 

 few and scattered. On a dry slope or bank along the roadside, 

 the leaves are decidedly thicker, and the hairs stouter and very 

 much more abundant; while under intermediate conditions corre- 

 sponding variations have been observed. 



Oenothera is an extremely plastic plant, responding readily to 

 changed conditions of environment. Lconurus Cardiaca, Lepidium 

 virginicum, Capsclla Bursa-pastoris, and others also show some 

 variations, but not to the same extent as Oenothera. Verbascum 

 Blattaria, on the other hand, is glabrous no matter under what 

 physical conditions it may be growing. Occasionally, when grow- 

 ing on a dry bank along a dusty roadside, a few hairs may be 

 found on the ventral side of the midrib of the lower stem leaves and 

 upper rosette leaves. This plant is extremely rigid and does not 

 at all, or but slightly, yield to changing conditions of environment. 

 It is perhaps a good illustration of a congenital mesophyte. 



In studying the number and distribution of hairs, Oenothera 

 biennis, O. rhombipetala, Leonurus Cardiaca, Lepidium virginicum, 

 Capsella Bursa-pastoris, and Hieracium paniculatum were selected 

 as types. Care was taken to collect both the stem and rosette 

 plant of each species in the same or as nearly the same habitat as 

 possible. Five plants of each species were studied, and the counts 

 for each particular kind of hairs were averaged and tabulated. 

 The field of the low power of the microscope was adopted as the 

 unit area of observation, and the average of 5 or more counts was 

 taken as the number for each area under observation. 



