April lo, 1884] 



NA 'JURE 



553 



tion, and were carried on chiefly during the months of 

 June, July, and August on a large number of Dicotyle- 

 dons from various families. Some t«enty-t\vo years ago 

 Prof. Sachs showed that the presence of starch in 

 chlorophyll grains can readily be dete.ted by means of 

 the now well-known iodine test, a modification of whi.'h 

 was employed in these researches. 



If fresh green leaves are plunged into boiling water for 

 ten minutes or so, certain soluble substances are extracted, 

 but the starch and colouring matter of the chlorophyll 

 grains remain in the still unbroken cells of the mesophyll. 

 A short immersion in alcohol now removes the green 

 colouring-matter and certain bodies soluble in alcohol, 

 leaving the starch behind in the colourless tissue. The 

 presence of acids afte.ts the degree of whiteness of the 

 decolorised leaf ; and the decolorisatioa proceeds more 

 rapidly in sunlight or warm alcohol than in the dark and 

 cold Leaves of Tropaolum may be rendered completely 

 white, like writing paper, in two or three minutes. 



If the decolorised leaf be now placed in a strong solu- 

 tion of iodine in alcohol, the presence or absence of starch 

 may be demonstrated in a few minutes. If no starch is 

 present, the cellular tissue simply prctents the well-known 

 yellow colour ; if a large quantity of starch exists in the 

 ceils, the tissue appears blue-black, the venation appear- 

 ing as a pale network in the dark ground. Pa'er colours 

 result if but little stirch is present at the time of the 

 c.\petinient. 



It will readily be seen how useful the above method is 

 for the purpose of demonstrating the absence of starch 

 from etiolated leaves, the white port ons of variegated 

 foliage, &c , and the sequel shows that the method affords 

 means of obtaining far more delicate results, without the 

 trouble of a microscopic e.xamination. 



In the first place, the same leaf may be found to con- 

 tain very different quantities of starch at different periods 

 of the day, or according to the weather ; and secondly, 

 the increase and decrease of the quantities of starch in a 

 given leaf may be very rapid. 



Sachs showed long ago that if a plant is pla:ed in the 

 dark, the starch disappears from the leaves ; and it has 

 also been known for some time that if a piece of tinfoil is 

 placed on a leaf, the covered portion forms no starch, 

 although the parts e.xposed to light may become filled 

 with that substance. Moreover, Krau5 showed how very 

 rapidly starch can be formed in direct sunlight. 



Sachs now demonstrates on a number of plants that 

 the starch formed in the leaves during the day may 

 disappear completely during the night, and that the leaves 

 shown to be full of starch in the evening may be quite 

 empty of starch next morning. This depends upon the 

 temperature and health of theplint, but occurs normally 

 during the summer in plants growing in the open. A 

 large number of experiments are given in support of this, 

 and showing how the rapidity and completion of the pro- 

 cess depends upon the weather. 



The experimental proof is very simple. A leaf is 

 halved longitudinally at night, after a fine sunny day, and 

 the excised half is shown to be filled with starch by the 

 iodine test described ; the remaining lulf is tested early 

 next morning, and shows at once if any material diminu- 

 tion has occurred during the night. A simple and obvious 

 modification of this experiment gives an idea of the 

 quantity of starch formed between sunrise and sunset. 

 The half leaf tested before sunrise shows no trace of 

 starch : the other half, left on the plant during the day, 

 is found to become more and more filled with starch 

 towards the afternoon. 



Some curious results were arrived at as to the effect of 

 growing parts on the rapidity of the emptying of the 

 leaves ; some of these matters still require investigation. 



Differences in the weight of leaves and in the intensity 

 of the colour produced by the iodine test, as well as some 

 other observations, lead to a better umierstanding of a 



fact already known generally, viz. that the starch disap- 

 pears from the leaves in the forai of glucoses, which 

 travel by way of the vascular bundles into the stems, and 

 thus pass to the places where they are used up in 

 growth. 



Some very telling observations were made in this con- 

 nection, and the dependence of the processes on tempera- 

 ture again show forth clearly. 



These results lead to the conclusion that the process 

 of metamorphosis into glucoses and translocation of the 

 products of assimilation are also going on during day- 

 light, though they are less evident, bjcau^e more starch 

 is then being formed and accumulated than is abstracted 

 at the time. Moll proved that such is the case by expos- 

 ing leaves to the sunlight, but in an atmosphere devoid 

 of carbon dioxide ; the starch already in the leaves disap- 

 peared, and no more was formed to replace it. Sachs 

 repeited Moll's experiments, and proved the correctness 

 of his conclusions by means of the iodine test. Half 

 leaves were shown to be full of starch ; the companion 

 halves were put into closed atmospheres, deprived of 

 carbon dioxide by means of potassium hydrate, and 

 exposed to sunlight. In an hour the latter halves were 

 tested, and found to be nearly emptied of starch. Other 

 experiments proved that depletion occurred in a few 

 hours, the time depending on the temperature. 



Further experiments demonstrate that the starch 

 travels in the form of glucoses in all the above cases ; but 

 it is not proved whether the metamorphosis is effected 

 by forces in the chlorophyll grains themselves, or by 

 means of diastitic ferments in the cells of the leaf. A 

 few hints are here given showing a field for further 

 research. 



Perhaps the most ingenious part of the paper is that 

 which now follows. It is well known that Weber's 

 patient and thorough researches on the energy of assimi- 

 lation led to two important results, ainong others : (i) 

 that the quantity of starch formed by a certain area of 

 leaf-surface in a certain time may be relatively very 

 large ; and (2) that different plants probably differ specifi- 

 cally as to the quantities of starch formed in their leaves. 



Sachs pro|)oses to apply his method to the solution of 

 this question, i.e. how much starch is produced in, say, 

 one square metre of leaf-surface by assimilation during, 

 say, ten hours' bright sunlight t The great difficulties in 

 Weber's researches were connected with the enormous 

 labour necessary to measure the leaf-su'-face accurately. 



Sachs resolved the matter in a manner nhich we may 

 summarise thus : — He cut off portions of large leaves 

 found to be empty of starch, measured them rapidly by 

 laying them on pieces of board cut to the size of one 

 square metre, and killed, dried, and weighed the mea- 

 sured portions very rapidly. Certain precautions as to 

 the area of fibro-vascular bundles, the possibilities of 

 absorbing hygroscopic moisture, &c., may here be 

 passed over. Sup osing the:e portions of the leaves 

 to be estimated in the morning, a quantity of the 

 same leaves of equal area gathered in the evening was 

 then compared, and the increase in weight gives the 

 quantity of starch formed in the interval. By weighing 

 large areas, and frequently, and by payirg attention to 

 the times and other circumstances, a large number of 

 results were obtained, showing that the quantities given 

 by Weber, for instance, are within the mark. ( f course 

 these results are not absolute. Starch is teing changed 

 into glucose, and passing away during the day, and some 

 must be burnt off in respitation ; moreover a certain 

 minute quantity of mineral ash should be allowed for. Of 

 course, it is an assumption that equal areas of mesophyll 

 of the same leaves contain approximately the same 

 amount of substance : nevertheless, if a large number 

 of experiments are made, the error is probably small. 



Experiments were made to show both the quantities of 

 starch which disappear during the night and the quanti- 



