PHOTOHARMOSE 



135 



breathing pores. The disposition of air spaces has much to do with the 

 arrangement of chloroplasts in both paUsade 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 api>earance 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- 

 drates decreases assimilation, and finally 

 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. 



Fig. 39. Position of chloroplasts 

 in aerial leaf (1) and submerged leaf 

 (2) of Callitriche bifida. X250. 



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 



