INDIVIDUAL AND SPECIFIC PECULIARITIES 357 



own climate most plants seem to have found it preferable to form new 

 leaves each year, rather than to acquire such resistant powers as would 

 enable them to survive over winter, but it is certainly always of importance 

 that the new leaves both of trees and of seedlings should rapidly acquire 

 the power of assimilating carbon dioxide, so that as little as possible of the 

 favourable period for vegetation may be wasted. Since the functional 

 activity of adult organs diminishes with age, it is evidently also of advantage 

 that even in evergreens the leaves should ultimately be thrown off and 

 replaced by new ones. 



The activity with which carbon dioxide is assimilated depends not 

 merely upon the illumination, or upon the position of the chloroplastids, 

 but also upon their specific assimilatory energy, for there is no doubt that 

 this varies in different chloroplastids and does not bear any direct or con- 

 stant relation to the amount of chlorophyll present l . Hence also while the 

 number of chloroplastids present in a leaf is an important factor in deter- 

 mining its assimilatory power, it is not the sole one, nor does the assimila- 

 tory energy of equal leaf-areas bear any constant relation to the number 

 of active chloroplasts in them. It is, moreover, easy to see that if the 

 assimilatory activity is measured per unit of weight, fleshy leaves will 

 appear to have abnormally weak assimilatory powers 2 . Hence in each 

 special case the combination of factors which influence the energy of assimi- 

 lation must be taken into consideration, and if the evolution of oxygen 

 or the increase of dry weight is used as a test for the amount of assimilation, 

 then in all cases the loss due to respiration must be calculated. 



Weber grew plants under constant conditions in a greenhouse and found that 

 after forty-eight days a plant of Phaseolus inultiflorus had gained 5-836 grammes in 

 dry weight, one of Helianthus annuus 29-806 grms., so that, allowing for the loss by 

 respiration, in ten hours a square metre of leaf-surface of Helianthus was able to 

 assimilate 5-559 gn~ns., of Phaseolus, 3-413 grms. of carbon dioxide. Haberlandt 

 calculated that a similar relationship existed between the number of chloroplastids 

 present in the same area of leaf-surface (approximately 495,000 per sq. m. in 

 Helianthus to 283,000 in Phaseolus\ but such a relationship need not necessarily 

 exist. The same criticism applies to the researches of Hansen, who found that 

 different plants yielding 3-9 to 5-9 grms. of pure chlorophyll per sq. m. of leaf, pro- 

 duced under similar conditions about i grm. of starch per 0-2 grm. of chlorophyll \ 



Since in feeble light but little carbon dioxide can be assimilated, and 



1 Cf. Sects. 52, 53, and 60. Examples of chloroplastids with different' specific assimilatory 

 powers are given by Engelmann, Bot. Zeitung, 1888, p. 718. 



2 For examples see Aubert, Rev. gen. d. Hot., 1892, T. IV, p. 440. 



3 Weber, Arb. d. Bot. Inst. in Wiirzburg, 1879, Bd. n, p. 350. Cf. also Geneaud. Lamarliere, 

 Compt. rend., 1891, T. CXIII, p. 230; Haberlandt, Jahrb. f. wiss. Bot., 1882, Bd. XIII, p. 95; 

 Hansen, Arb. d. Bot. Inst. in Wiirxburg, 1887, Bd. Ill, p. 428. 



