20 Varietäten etc. — Physiologie. 



have largely dealt with modifications, and the conclusion reached 

 that little or no advance could be expected from the selection of 

 vegetative variations. There is certainly good evidence accumula- 

 ting, as the werk of Sbamel shows, that among citrus fruits at 

 least, distinct advance is being made in plant improvement through 

 bud selection alone. This is no doubt a case in which true segre- 

 gates or mutants have been utilized, and, if so, they can be expected 

 to remain constant under a wide ränge of conditions. There is a 

 broad field for work on the genetics of bud-variations and when 

 their nature is better known and the lines of discrimination more 

 clearly drawn between the several classes, more rapid advance in 

 their utilization may be expected. It seems likely that many of the 

 laws pertaining to the perpetuation of fluctuations among semi- 

 nsLlly propagated pure lines will apply with equal force to vegeta- 

 tive fluctuations in a clonal variety. If so, a study of the one may 

 well be taken into consideration in connection with a study of the 

 other in any attempt to arrive at the fundamentals underlying 

 either. And it seems entirely possible that a closer study of the 

 cases of vegetative segregations and mutations, as they come to 

 hand, will aid materially in the advancement of knowledge concer- 

 ning the nature of Variation in general and the discovery of the 

 principles underlying it. M. J. Sirks (Wageningen). 



Graves, H. A., Chemotropism in Rhisopus nigricans. (Bot. Ga- 

 zette. LXII. p. 337—369. 4 Fig. 1916.) 



The foUowing results apply to Rhisopus nigricans Ehrenb. in 

 particular, but experimental evidence is at hand that the general 

 principles involved apply also to Botrytis cinerea Pers. and Penicil- 

 lium W 24 Thom. Most of the data have been derived from work 

 with two layers of medium separated by a perforated mica plate. 



The fungus shows a marked negative chemotropic reaction to 

 a medium in which it has been growing for some time. 



The hypothesis brought forward by Clark and Fulton, that this 

 negative chemotropism is a reaction of the fungus toward it „sta- 

 ling substances" is substantiated by the behaviour of the hyphae. 



These staling substances are formed as a result of the vital 

 activities of the fungus itself. They consist probably of excreted pro- 

 ducts of metabolism. 



The staling substances appear to be either thermolabile or vola- 

 tile, for boiling a Solution containing them reduces markedly their 

 negative chemotro'pic influence. 



Positive chemotropism toward the substances tested (turnip juice, 

 cane sugar, and glucose) also exists; but under ordinary conditions 

 of growth this positive chemotropism is very much weaker than 

 the negative chemotropism previously mentioned. 



Turnip juice exerts a much stronger positive chemotropic Sti- 

 mulus than the other simple chemical substances tested, much 

 stronger than, for example, 5 per cent cane sugar. This suggests 

 the possibility that plant juices in general evoke a stronger positive 

 chemotropic response than the simple chemical substances hereto- 

 fore experimented with. The nature of the attractive substance or 

 substances in turnip juice has not yet been determined. 



It is impossible in the present stage of our knowledge to com- 

 pare accurately the strength of two chemotropic Stimuli; but, using 

 the number of hyphae turning as a test of the relative action of the 



