PHOTOHARMOSE 135 
breathing pores. The disposition of air spaces has much to do with the 
arrangement of chloroplasts in both palisade and sponge tissues. Starch 
formation is also dependent upon the presence of air spaces, but, con- 
trary to what would be expected, it seems to be independent of their size, 
since sun leaves, which assimilate much more actively than shade leaves, 
have the smallest air spaces. From this fact, it appears that the rapidity 
of aeration depends very largely upon the rapidity with which the gases are 
used. Translocation likewise affects the arrangement of the chloroplasts 
and the formation of starch. According to Haberlandt, it also plays the 
principal part in determining. the form 
and arrangement of the palisade cells. 
Chloroplasts are regularly absent at those 
points of contact where the transfer of 
materials is made from cell to cell, though 
this is not invariably true. Since air pas- 
sages are necessarily absent where cell 
walls touch, it is possible that this disposi- 
tion of the plastids is likewise due to the 
lack of aeration. Translocation is directly 
connected with the appearance of starch. 
As long as all the sugar made by the 
chloroplasts is transferred, no starch ap- 
pears, but when assimilation begins to 
exceed translocation, the increasing’ con- 
centration of the sugar solution results in 
the production of starch grains. The 
latter is normally the case in all flowering 
plants, with the exception of those that 
form sugar or oil, but no starch. The 
constant action of translocation is practi- 
cally indispensable to starch formation, 
since an over-accumulation of carbohy- , Fig.39. Position of chloroplasts 
drates decreases assimilation, and finally (5 seta Tee) aruba rete leat 
L , Y (2) of Callitriche bifida. « 250. 
inhibits it altogether. In consequence, 
translocation occurs throughout the day and night, and by this means the 
accumulated carbohydrates of one day are largely or entirely removed 
before the next. 
176. The measurement of responses to light. Responses, such as the 
periodic opening and closing of stomata, which are practically the same for 
all leaves, are naturally not susceptible of measurement. This is also true 
of the transpiration produced by light, but the difficulty in this case is due 
