RECENT ADVANCES IN SCIENCE 51 



when the piece of stem is cut out from the whole plant does 

 not grow out as long as the piece is part of a whole and normal 

 plant, the author suggests that the growing apex as well as the 

 leaves of a plant continually produce and send towards the 

 base of a plant substances which inhibit the growth of dor- 

 mant buds. When a piece of stem is cut out, the inhibitory 

 substances then present in the stem will continue to move 

 towards the base, so that the most apical buds will be the 

 first to become freed from the inhibitor, with the result that 

 they are the first to grow out into shoots. As soon as this 

 happens these latter send out the inhibitory substances, as a 

 consequence of which none of the more basally situated buds 

 are able to develop further. 



Similar considerations are presented in regard to the 

 phenomena of geotropic curvature in Bryophyllum, where the 

 author assumes that a hypothetical geotropic hormone is 

 associated, or identical, with a root-forming hormone, which is 

 responsible for the geotropic growth which takes place in the 

 cortex of the lower side of a horizontally suspended stem. 



A somewhat different outlook, which perhaps approaches 

 the happy medium between the two extreme views cited 

 above, and which perhaps best illustrates the modern ten- 

 dencies in plant physiological research, is found in a paper by 

 G. Gassner (" Beitrage zur physiologischen Charakteristik 

 sommer- und winterannueller Gewachse insbesondere der 

 Getreidepflanzen," Zeitsch. f. Bot. 191 8, 10, 417-80). The 

 general characteristic of winter and summer annual plants is 

 the length of the vegetation period, winter annual plants ger- 

 minating the autumn of the year preceding that of maturation. 

 A closer experimental examination reveals that the conditions 

 are not so simple. Winter annual plants can be brought to 

 germinate in the spring of the same year in which maturation 

 occurs, for instance, by allowing the seeds to germinate at 

 a temperature of + i° to + 2 C. and then transferring them 

 to the open. One cannot, therefore, say that the characteristic 

 of a winter or a summer annual plant is due to some hereditary 

 property independent of external conditions. The life period 

 under natural conditions is determined by a certain co-opera- 

 tion between certain internal characteristics and external 

 natural conditions. We cannot thus speak of a specific 

 autonomous life period of annual plants, nor of a rhythmic rest 



