VARIATION 



33.-) 



remained constant for hundreds of test-tube "generations." It must 

 be admitted that in most of these cases no specific influences can be 

 named as the direct cause of the inherited variation. But there is no 

 longer any doubt that permanent, discontinuous variations do occur 

 spontaneously in these lowest organisms, and it is highly probable 

 that certain incidental, external forces play an important part in 

 inducing such variations. 



Direct experimental attack upon the germ cells themselves has 

 been made with plants by a number of investigators, notably by 

 MacDougal, who injected very dilute solutions of potassium iodide, 

 zinc sulphate, sugar, etc., directly into the ovaries of various plants 

 immediately before fertilization. Consequently somatic changes have 

 been produced which were inherited throughout several generations. 



Fig. 56. — O, portion of leaf of Scrophularia showing branching lateral vein; 

 D, branching vein replaced by two laterals in leaf of seedling grown from seed 

 produced by an injected ovary. Also note the difference in size and margin of 

 leaves. {From Babcock and Clausen, after MacDougal.) 



By means of check experiments and observations it was found that 

 these germinal variations were not caused by the wounding of the 

 ovary and it is thought that they must have been induced in some way 

 by the presence of the foreign chemical solution in the ovary. Fig. 56 

 shows a morphological change which appeared in a seedling of an 

 unnamed species of Scrophularia as a result of ovarial injection. Hav- 

 ing tested the species sufficiently to determine that it was a simple one, 

 MacDougal treated several ovaries with potassium iodide, one part 

 in 40,000 and secured seed. No other species of Scrophularia grew 

 near the cultures. From this seed only three plants were raised. 

 "One formed a shoot fairly equivalent to the normal, finally producing 

 flowers in which the anthocyans were of a noticeably deep hue. The 

 two remaining plantlets were characterized by a succulent aspect of 

 the leaves and by a lighter and yellow color of the leaves and stems. 



