IN VEGETABLE BIOLOGY. 208 
spaces. To express the two conditions of perpendicularity to 
and parallelism with the incidence of light, Stahl coins the words 
“ Flüáchenstellung ” (corresponding with the “ Tagesstellung " of 
Borodin) and “ Profilstellung" (the analogue of “ Nachtstellung”). 
This author shows that if leaves of mosses and fern prothallia be 
brought from darkness into direct sunlight, no change in the 
position of their chlorophyll ensues, but that, after insolation, 
removal to the dark causes the grains to shift, to a greater or less 
extent, from Profilstellung into Flächenstellung. The chief point 
brought out by Stahl, however, is the one so well known from the 
reproduction of the figures in Sachs’s and Pfeffer’s works on 
Vegetable Physiology, viz. the behaviour of the chlorophyll of . 
Oxalis Acetosella * leaves set in direct sunlight, under which 
circumstances the grains of the two lower layers of mesophyll 
cells first pass into Profilstellung and afterwards mass in the cells’ 
corners, just as Borodin showed to be the case with Lemna trisulca 
&c. Stahl compares these phenomena with the movements of 
Zoospores and Desmids f; his memoir also deals with several 
allied subjects which have no bearing upon the present paper. 
The terminology adopted in the following pages is that of 
Frank, as being in best aecord with the genius of our language. 
By the word “ Epistrophe," however, I wish to be understood 
as implying the arrangement of the chlorophyll grains upon 
those walls which are at right angles to the plane of incident 
light; by “ Apostrophe ” the setting of the grains upon walls 
parallel to that plane. Moreover, it has been explained that 
apostrophe is caused by absence as well as by excess of light, and 
I shall show that it may sometimes be induced in poor grades of 
illumination ; when occurring in the dark or in feeble light it is 
proposed to qualify the apostrophe as “negative,” while “positive” 
apostrophe will express that the re-arrangement is the result of 
exposure to high illumination. To the whole phenomenon the 
term “ photolysis " may perhaps be deemed suitable. l 
Are the grains drawn passively along with the streaming plasma, 
or have they the faculty of independent motion? is a question 
which has often been asked. It was answered years ago and re- 
peatedly since in the former sense by Sachs f, and Frank $ was 
* Stahl also saw the same thing in many other types. . 
+ Strasburger had previously done this in his memoir on zoospores in the 
' Jenaische Zeitschrift’ for 1878. 
t ‘Lehrbuch’ and No. xxxv. of the ‘ Vorlesungen.’ 
$ Pringsheim’s Jahrb. f. wiss. Bot. viii. pp. 294 and 282. 
