1909.] 



Vegetable Assimilation and Respiration. 



9 



some extent, especially with Hciiantlms, to shrinkage of the experimental 

 half-leaf ; for the leaf substance, as a whole, contains about 50 per cent, of 

 carbon, whereas starch contains only 44 per cent. 



Only in one instance is the difference too great to be easily explained 

 (Table I, Leaf 2). Here it is possibly due to an exceptional experimental 

 error. 



Composition of the Gain of Dry Substance. 



Some further points of interest connected with these analyses require con- 

 sideration. In the first place, they suggest inorganic substance as a possible 

 constituent of the gain of dry weight. It is not certain how far the observed 

 changes in ash content were real, or how far only apparent and due to 

 shrinkage ; but differences of 12 or 13 per cent. {cf. Table II) are far too 

 large to be reasonably accounted for by shrinkage. Moreover, an accumu- 

 lation of inorganic substance is to be expected, at any rate in leaves attached 

 to the plant and in detached leaves supplied with nutritive solutions.* It is 

 also probable that these substances are translocated. 



Broockst and MenzeJ have also published ash analyses which tend to 

 support these suggestions, although, again, the interpretation of their results 

 is rendered somewhat doubtful by the possible occurrence of shrinkage. 

 Hence, until more conclusive experiments have been carried out, it is 

 necessary to bear in mind that changes of dry weight may consist in part 

 of ash, perhaps to the extent of 5 per cent, or more, and in using Sachs' 

 method it is advisable, for accurate work, to determine the ash content of 

 the dried leaf material and deduct it from the dry weight. § 



Having thus obtained the increase in organic substance, there still remains 



* The analyses in Table II suggest that even when detached leaves are supplied with 

 distilled water an accumulation of mineral substance in the lamina may take place, 

 presumably by translocation from the stalk and principal veins. 



Menze's ash analyses may also indicate a similar translocation : he used detached leaves 

 of trees like Platanus, Tilia, Quercus, exposed them to diffused light only, and protected 

 them from wind by inverted beakers, so that it is more probable that there was some 

 expansion than that much shrinkage took place. In the case of those leaves which were 

 exposed in closed vessels of air free from carbon dioxide, the apparent fall in ash content 

 observed was probably due to a positive area change. 



t Broocks, W., " tlber tiigliche und stlindliche Assimilation einiger Kulturpflanzen," 

 'Inaug.-Diss.,' Halle, 1892, pp. 17—20. 



I Menze, O., "Zur Keuntniss der taglichen Assimilation der Kohlehydrate," 'Inaug.- 

 Diss.,' Halle, 1887, pp. 10—12. 



§ The ash content in Table III for Cherry Laurel shows irregular fluctuations ; but 

 the quantities dealt with were small, and the changes themselves scarcely exceed, in 

 most cases, possible experimental errors. Cherry Laurel transpires but slowly, and a 

 large increase would therefore not be expected. 



