775 



acts in definite quantity on that organ uniformly from all sides. 

 The first results of such an inquirv with i-espcct to liftht will now 

 be described. 



For various reasons I have chosen in the (irst place the sporan- 

 giophores of Phycoinyces as objects for investigation. The most im- 

 portant of these reasons was, that it was obviously desirable first to 

 trace the influence of light on a single cell and only then to in- 

 vestigate it in multicellular organs such as the stem and the root 

 of higher plants. 



I have postponed for later more detailed description an account 

 of the arrangement for securing a constant temperature, of the method 

 of cultivation and of other details. I will here only mention, (hat 

 the fungus was grown at the same constant temperature at which 

 the experiment was afterwards carried out. During the experiments 

 the temperature remained constant within ^l^° C. I found that a 

 rapid rise of temperature of, for example, only 7in° ^^ «lay exercise 

 a considerable influence on the rate of growth of the sporangio- 

 phores, if it be only for a few minutes. 



It can be noticed, that with the small, but sudden rise of Vio° C. 

 the rate of growth may for a short time decrease by as much as 

 257o, and then only rise again to the normal. In this case it is 

 perhaps not the temperature itself, which directly affects growth, 

 but a brief change in the degree of humidity of" the atmosphere 

 round the plant. I hope to deal with this more in detail. The chief 

 point here is to show, how important it is to attain a high degree 

 of constancy in experiments of this nature. 



In the experiments sporangiophores are used, which are three to 

 four cm. high. It is known from Errera's investigation {Bot. Zeitum/ 

 42^'' Jahrg. 1884) that they are then in a condition, in which they 

 possess a maximal and practically constant growth. 



The sporangiophore employed, is placed in the centre of a box, 

 which remains therefore at a very constant temperature and in which 

 the atmosphere has a rather high degree of humidity, which throughout 

 the experiment remains quite constant. The growth is observed through 

 a double plate of thick glass, by means of a telescope placed outside 

 the box and magnifying 40 times. The light for the observation was 

 obtained from a weak, red lamp, which was switched on only during 

 the observation, for as short a time ag possible; its feeble light, also 

 passing through a double glass plate, forms a silhouette of the plant 

 on a red background. The illumination of the i)lant, placed in a 

 central position, is carried out by allowing light to fall from above 

 through a double plate of glass. Whilst the plant is prevented from 



50 



Proceedings Royal Acad. Amsterdam. Vol. XVI. 



