172 ON BIASTREPSIS IN ITS RELATION TO CULTIVATION. 



vent the crossing of two varieties of the same species if in flower at 

 the same time. 



Conclusions. 



1. The seed of Dipsacus sylvestris torsus yields, under proper 

 cultivation, a progeny of which about one-third have twisted stems. 

 This proportion was first attained in the fourth generation, and since 

 then it has rather increased than diminished on the whole (see p. 160). 



In addition, there occur plants with three-leaved whorls, with 

 divided leaves or with local twisting of the lateral branches and 

 occasionally the other anomalies which I have described in my 

 monograph^). 



2. The development of biastrepsis, that is,* the transition from 

 decussate to spiral phyllotaxis, depends not only upon the hereditary 

 properties of the individual latent in the seed, but also in a high 

 degree upon the external conditions under which the individual 

 develops. 



3. The more favourable the conditions of life and consequently 

 the more vigorous the growth of the plants, the richer is the progeny 

 obtained from any given seed in individuals with twisted stems and 

 the more marked is the twisting in the individuals. 



4. The most important condition is that the plants shall have 

 plenty of space for their growth: they must not overshadow each 

 other and they should touch each other as little as possible. There 

 ought never to be more than 20-25 plants to the square metre; and 

 even then the plants come into contact with each other in the au- 

 tumn: it would be better not to have more than 10-15 plants per 

 square metre. 



When too closely planted, the number of plants with twisted stems 

 per square metre is less than that obtained with more remote plan- 

 ting; so that the greater number of individuals is not an advantage 

 but a disadvantage. In the case of close planting the twisted indi- 

 viduals are confined, either exclusively or for the most part, to the 

 border of the bed. 



5. The time of sowing is of importance, for this determines the 

 length of life of the plant up to the time when the shoot is thrown 

 up: the longer this period, the conditions being favourable, the 

 greater is the prospect for biastreptic individuals. 



i) In addition to the references given on pp. 154 and 155, 1 may 

 mention the following papers which 1 have published on this subject: 

 Oj>era V, p. 206, 407 and 478. 



