ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



481 



constitution of the plant, wLicli is manifested by the quantity and 

 quality of the dry substance. All external factors which appear to 

 exercise a direct influence on transpiration, affect the quantity, distri- 

 bution, and composition of the dry substance, and transpiration only 

 indirectly through these. The most important factor in transpira- 

 tion is light ; and the transpiration which takes place by night is a 

 secondary result of the light during the previous day, as is shown by 

 the fact that the amount of transj)iration is often greater during the 

 night than during a similar period in the following morning. 



Formation of Starch-grains.* — Professor A. F. W. Schimper 

 publishes the results of an important series of observations on the 

 mode of formation of starch within the chlorophyll-grains. Under 

 ordinary circumstances, the starch-grain is formed in all portions of 

 the chlorophyll-grain. This is the case in the mesophyll of the leaf, 

 and also in the green parts of the stem of some, but not of all, flower- 

 ing plants. But in the stem of many plants the starch-grains are 

 formed in a diflerent way to this, viz. only immediately beneath the 

 surface of the chlorophyll-grain, from which they escape by breaking 

 through the thin layer that overlays them. When the chlorophyll- 

 grains are spherical or flattened, these starch-grains are formed in all 

 parts of the periphery ; but when the former are disk-shaped, the 

 localization is carried still farther, and the formation of starch-grains 

 is confined to their equatorial zone. 



A diiFereuce in the structure of the starch-grains is intimately 

 associated with the locality of their formation. Those produced in 

 the interior of chlorophyll-grains, and entirely surrounded by the 

 chlorophyll, as in the parenchyma of the cortex and pith of Cereus 

 speciosissimus,h?iYQ a centric structure, but usually remain very small; 

 while those formed near the periphery are much larger, and eccentric 

 in their structure, the side which grows most rapidly being the one 

 immersed in the chlorophyll ; and the unequal growth is therefore 

 evidently due to unequal nutrition. This is well exemplified in the 

 starch-grains formed in the stems of many plants. 



In those parts of the plant which contain starch but no chloro- 

 phyll, the grains are not surrounded by ordinary protoplasm, but are 

 enclosed in peculiar strongly refractive particles, usually of spherical 

 or fusiform shape, which are remarkably unstable, disappearing as 

 soon as fluid enters the cell. These particles are produced before the 

 starch-grains, and these latter are formed within them either in all 

 parts or near the periphery only, and their structure then varies 

 in precisely the same way as that of those formed within chlorophyll- 

 grains ; the former are centric, the latter eccentric. These albuminous 

 particles are unquestionably the organs for the formation of starch in 

 those cells which do not assimilate, that is, they transform into starch 

 the assimilated substances conveyed to them from other parts of the 

 plant, and may hence be termed starch-producers {^t-AvkGh\\(iniiv). The 

 development of these starch-producers, and of the starch-graius within 



* Bot. Ztg., xxxviii. (1880) pp. 881-902 (1 pi.). 



