18 



dry ground with garden-cultures, and found that with the 

 plants from the greenhouse the médian of the length of 

 the leaves was only about half that of the plants that 

 had grown in full ground, the average riumber of ray- 

 flowers per head being 12 with the former, 13 with the 

 latter. In his work „die Mutationstheorie" de Vries') 

 describes experiments and observations, the chief object of 

 which has been the comparison of the influence of nutri- 

 tion with that of sélection, but which at te same time 

 increase our knowledge .about the influence of nutritive 

 conditions as such. He investigated the influence of thèse 

 two factors on the length of the fruit of Oenothera Lamar- 

 ckiana and Oenothera rubrinervis, on the number of umbel- 

 rays of Anelhum graveolens and Coriandrum sativum and 

 on the number of ray-flowers of Chrysanthemum segetum, 

 Coreopsis tinctoria, Bidens grandiflora and Madia elegans. 

 From his observations de Vries concludes that nutrition 

 and sélection act in the. same direction and that by stron- 

 ger nutrition as well as by positive, sélection the médian 

 value of a character is increased. Moreover he generally 

 observes that the variability of the characters is increased 

 when nutrition and sélection act in opposite directions, 

 1. e. when, as in his experiments, strong nutrition goes 

 together with négative sélection. 



Also the experiments by Keinôhl") on the variability 

 of the number of stamens of Stellaria média show that 

 with good nutrition the médian of this character possesses 

 a higher value than with bad nutrition. Besides Reinôhl 

 finds that the index of variability, which is a measure for 

 the variability, becomes smaller under unfavourable nu- 

 tritive conditions. 



1) H u g o d e V ri e s, Die Mutationstheorie. Bd. I. p. 308. 



2) Friedrich Reinôhl, Die Variation ira Androceum der 

 Stellaria média Cf/r. Bot. Zeit. 1903, p. 159. 



