Variations and Mutations 



281 



stems and leaves and in the chapter dealing with environ- 

 mental factors. 



Fig. 179. Varieties derived from the wild cabbage (F), a native plant of Europe. 

 A is kohl-rabi, B cabbage, C cauliflower, D kale, and E Brussels sprouts. 



Temperature, moisture, light, mineral salts of the soil, all have 

 effects upon the plant. Consequently, when plants having the 

 same hereditary qualities are grown in dissimilar habitats, the 

 different environment brings about marked variances in the ex- 

 pression of those qualities. Variations of this kind are not 

 inherited, although when the vigor is decreased by the conditions 

 in which the plant is grown, the plants of the next generation 

 may get a poor start by less vigorous seedlings and show at matu-. 

 rity some of the effects of unfavorable conditions to which the 

 previous generation was exposed. 



Fluctuations. Variations due to differences of the environment 

 are often called fluctuations, and we can now associate many of 

 them with the particular external factors which produce them. 

 Another class of fluctuations are those which appear to be due to 

 unknown internal causes. The leaves that occur on a mulberry 

 tree, for example, may vary from leaves that are almost perfectly 

 heart-shaped to those with severahlobes. The number of leaflets 

 that makes up a compound leaf of the horse chestnut, walnut, 

 ailanthus, and sumac varies somewhat widely. In a California 

 privet hedge one finds branches usually with leaves opposite, 



