Variation occurring in Biscutella Isevigata. 21 



by one individual exhibit a fairly uniform degree of hairiness (or 

 smoothness) are almost invariably those belonging to the extreme 

 types ; the plants are either very hairy (type I) or almost glabrous 

 (types IIIc and the smoother forms of III6). All the plants in 

 which the leaf surface was originally free from hairs remained 

 smooth, while out of the total number of hairy plants only two 

 varied from the original type. It is those in which the first formed 

 leaves are intermediate in character between these two extremes that 

 the change from a more to a less hairy condition may generally be 

 traced (types II, Ilia, and the hairier forms of III6) . Consequently 

 we find that the continuous series of gradations from the condition of 

 absolute smoothness to that of extreme hairiness, which may be 

 observed upon examination of the leaves of a large number of seed- 

 lings taken at random, is not met with in an equally large and hap- 

 hazard collection of adult plants ; among the latter certain forms 

 have disappeared, and the types which obtain are more sharply 

 marked off from one another. In fact adult plants fall into two 

 groups ; the type with leaves with an intermediate surface is not 

 found, or occurs, as a rare exception. It follows, therefore, that a 

 census compiled from a set of adult plants, and a similar record 

 obtained from the same individuals before the stationary point has 

 been reached will not give concordant results ; in the former case the 

 proportion of plants bearing leaves with a glabrous surface will be 

 higher than in the latter. 



In order to ascertain the nature and amount of the variations 

 occurring among the offspring of unlike parents, certain individuals 

 which flowered in the summer of 1896 were intercrossed. The plants 

 were placed under muslin covers in order to exclude insects, and the 

 flowers were emasculated while still in bud before the anthers had 

 dehisced, in order to prevent possible self-fertilisation. The seeds 

 thus obtained were sown the same year after having been allowed to 

 ripen for a few weeks ; the character of the cross-bred seedlings is 

 shown in Tables VII and VIII. 



