RECENT ADVANCES IN SCIENCE 393 



The photo-electric cell of Elster and Geitel is not open to this 

 objection, and it is therefore suggested by the authors as a 

 more satisfactory instrument for plant physiological measure- 

 ments. Measurements of the same insolation made with the 

 two instruments show considerable differences. 



Undoubtedly some of the most important investigations 

 on the influence of environmental factors are those of Klebs 

 (Biol. Zentralbl. 37, 373—41 5, 191 7 ; Flora, 11-12, 128-51, 

 191 8). In the first of these papers cited, evidence is brought 

 forward in support of the author's thesis that growth and 

 rest periods in plants are ultimately the result of external 

 conditions. In the beech, for example, the normal develop- 

 ment of leafy shoots from the bud only continues for a few 

 weeks at most. By the employment of the continuous 

 illumination of an Osram lamp and by limiting the number of 

 illuminated shoots to one at a time, it was found possible to 

 make a small beech tree develop leafy shoots continuously for 

 eight months. In the same way an oak tree was made to 

 develop leafy shoots continuously for a period of four summer 

 months and three winter months. A large number of ex- 

 periments were also carried out with Ailanthns glandulosa, in 

 which the influence of environmental factors on development 

 was clearly shown. Among the more important points brought 

 out in this investigation are the fact that the development of 

 a young plant of Ailanthus glandulosa can be stopped by 

 gradually decreasing photosynthesis, or by decreasing the 

 content of mineral salts in the soil, while the rest period can 

 be brought to an end by bringing the plant into a warmer, 

 more humid and dark environment, by introducing the plant 

 into a soil rich in nutrient salts, or by continuous illumination. 



F. Weber (Ber. deut. hot. Ges. 34» 7-13, 19 16) opposes the 

 view of Klebs that under natural conditions the rest period 

 of the beech in winter is due to low light intensity, since potted 

 plants of the beech will unfold their leaves in midwinter when 

 exposed to an atmosphere of acetylene. Weber's argument 

 seems, however, to be beside the point. 



In the second paper of Klebs referred to above, the results 

 of an investigation into the causes of the transition from 

 vegetative growth to flower-production are recorded, the 

 plant investigated being a species of Sempervivum. It had 

 been found earlier that this transition takes place when 

 certain changes occur in the environmental conditions. For 

 instance, humidity accelerates growth and hinders flower- 

 production, and conversely intense transpiration acts in the 

 reverse way. Klebs had previously expressed the opinion 

 that the essential condition for flower-formation is the accumu- 

 lation of carbohydrates in the plant. Hence, increase in 

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