590 SUMMARY OF CURRENT RESEARCHES RELATING TO 



water for a few minutes, and then for a short time in alcohol. If 

 then placed in a strong alcoholic solution of iodine, the decolorized 

 leaf will be stained a buff-yellow if no starch is present, blue-black if 

 starch is present in great quantities, with intermediate shades according 

 to the amount of starch. 



The formation of starch is entirely dependent on the presence of 

 light ; and Sachs's experiments show that the starch formed during 

 the day may disappear completely during the night ; the cells of the 

 leaves being full of starch in the evening and quite empty in the 

 morning, when the conditions of temperature are favourable. It is 

 stated that the starch disappears in the form of soluble glucoses, 

 which travel, through the vascular bundles, to the parts where they 

 are wanted for purposes of growth. Although this process takes place 

 chiefly in the night, it is also going on more slowly through the day, 

 but is then masked by the much more energetic production of starch. 

 The transformation of starch into sugar may possibly be due to the 

 presence of a diastatic ferment in the cells of the leaf. 



By an ingenious contrivance the quantity of starch produced and 

 converted into glucose was approximately measured. In HeliantTius 

 annuus 4 • 64 grms. disappeared in ten hours from 1 sq. m. of leaf- 

 surface ; and in the same plant 9 • 14 grms. were formed in the same 

 time on the same area. In another case, where the leaves were 

 removed from the stem to prevent the return of the starch from the 

 leaf to the stem, a sq. m. was found to produce starch at the rate of 

 1 • 648 grm. per hour. As a general result, Sachs concludes that, in 

 ordinary circumstances, a leaf may produce in the day from 20 to 25 

 grms. of starch per sq. m. of surface ; and under certain conditions 

 it may even be larger. 



Quantitative Relation between Absorption of Light and Assimi- 

 lation.* — T. W. Engelmann gives the result of a number of observa- 

 tions on this point, accompanied by mathematical formulae. He 

 regards the bacteria-method f as far the most exact for the purpose. 

 The general results may be stated as follows : — The absolute minimum 

 of absorption lies in the outermost red. Between B and E, to the 

 highest degree at F, lie one or more maxima and minima. The 

 amount of absorption increases constantly, attaining its maximum in 

 the more refrangible part of the visible spectrum. The amount of 

 assimilation corresponds to the amount of absorption in all cases from 

 the outermost red to the green ; while in the more refrangible part 

 the amount of assimilation falls notwithstanding a regular increase 

 of absorption. 



Causes which Modify the Direct Action of Light on Leaves.j — 

 Pursuing this subject, E. Mer arrives at the following conclusions : — 



1. The position of leaves is not always an index of their direct 

 relation to light; for this sometimes results from influences which 

 modify more or less the direct action of light. 



* Bot. Ztg., xlii. (1884) pp. 81-93, 97-105 (1 pi.), 

 t See this Journal, iii. (1883) p. 390. 



X Comptes Rendus, xcviii. (1884) pp. 836-8. Of. this Journal, iii. (1883) 

 p. 386. 



