296 Physiologie. 



hier noch erbrachte Nachweis, dass die „Wanderzeit" der be- 

 weglichen Stärke stets erheblich kürzer als die Präsentations- 

 zeit ist. Noll. 



Mac Dougal, D. T., The Influence of Light and 

 Darkness upon Growth and Development. (Mem., 

 N.Y. Bot., Garden. Vol. II. p. I— XIII, 1—319. Fig. 1 — 176. 

 Jan. 20. 1903.) 



A very comprehensive study of the question, including 

 experiments and observations on no less than one hundred and 

 two different species of plants, representing many orders. A 

 historical summary of thirty four pages opens the work, after 

 which follow detailed descriptions of the effect of etiolation 

 on the extensive series of plants mentioned above. Under the 

 caption, General Considerations, the effect of etiolation on 

 practically all types of stems and modifications of stems is 

 discussed, and in the same section of the paper the histological 

 condition of the important categories of tissues finds treatment. 

 Then follows a description of the etiolated condition of leaves, 

 both of Monocotyledons and Dicotyledons, and later the effects 

 of darkness on flowers and inflorescences, on spores and 

 sporangia of ferns are taken up. Finally the author considers the 

 relation of fungi to Iight and darkness. 



In discussing the theories of etiolation and the morphogenic 

 effects of light and darkness the author says in part as follows. 

 Etiolation is not an adaptation to darkness and the forms which 

 plants assume in the dark are not governed by an effort to 

 reach the light. The various phenomena of etiolation are in 

 the first instance due to the mere absence of light and there 

 follows in some cases modifications which may be regarded as 

 benificial to the plant, but the modification may at times be 

 disadvantageous. Aside from the lack of Chlorophyll, the basal 

 fact connected with etiolated organs is that their tissues do not 

 show the same degree of morphological differentiation as do 

 the tissues of normally grown organs. A comparison of normal 

 and etiolated plants shows that growth and differentiation are 

 not only independant phenomena, but are easily separable. 

 The lack of differentiation and the augmentation of growth in 

 etiolated organs is most noticeable in parenchymatous tracts of 

 tissue. There is an incomplete deposition of aplastic material, 

 which allows of a much longer growing period. 



The phenomena of etiolation rest upon the absence of the 

 morphogenetic influence of light or rather of the influence of 

 alternation of light and darkness. Light acts, then, as a 

 stimulating influence in inducing morphological differentiation, 

 but it is not necessarily direct in action, since the stimulative 

 influence may be received by one portion of the body and 

 transmitted to another. The impulse may even be communicated 

 to organs not actually formed at the time. 



