1910.] Vegetable Assimilation and Respiration. 



437 



Cncurhita* may prove to be close rivals of Helianthus, and Weber's 

 resultsf suggest that Eiciims comvuinis and Tropceolum majus are not far 

 behind. 



Catalpa, on the other hand, is a tree, and although like Helianthus it is 

 comparatively large-leaved and grows fairly rapidly, its leaves have stomata 

 only on their lower surface, and therefore cannot be expected to show such 

 high rates of assimilation. It is not surprising, therefore, that in the 

 following experiments the highest rate of assimilation is far below corre- 

 sponding rates for Helianthus obtained in the experiments described in 

 Section II. 



The results given by individual leaves of Catalpa in the same experiment 



do not show the same agreement as was found with Helianthus; instead 



they vary within very wide limits, even when, as in Expt. 6, the apparent 



condition of the different leaves was remarkably uniform. This- is chiefly 



due to the high degree of asymmetry in respect of dry weight per unit area 



shown by the leaves of this plant. The following asymmetry tests by the 



stamping method form the basis of the asymmetry error estimated for 



Expt. 6, in which that method was used : — 



Area taken, 11"1 sq. cm.| 



Differences found, milligrammes per sq. dec... +21, +34, +26; average, ±27. 

 Percent +5"], +8, +6"7; average, ±6"4. 



Area, 19'8 sq. cm. 



Differences found, milligrammes per sq. dec... —18, —4.3, +3, +33, +14, —13 ; average, 



±21. 



Percent -4-3, -11-2, +07, +5-7, +2-2, -31; 



average, +4 "5. 



Area, 38"1 sq. cm. 



Differences found, milligrammes per sq. dec... +25"6, +8'3, — 1'7, +21'G, — 2'5; ave- 

 rage, ±11-8. 



Percent +4'6, +1, -0*4, +4-4, -0-5; average, 



±2-2. 



* Cf. Sachs' results for attached leaves of Cucurbita and Helianthus, loc. cit. 



t " Ueber specifische Assimilationsenergie," 'Arb. d. Bot. Inst. Wiirzburg,' vol. 2, 

 p. 346. See Sachs, 'Lectures on the Physiology of Plants,' 1887, p. 313. 



\ These results are, on the whole, comparable with those previously obtained by the 

 templet method (Thoday, loc. cit., p. 18 ; cf. also Brown and Escombe, loc. cit., p. 60). 

 One point of importance arises from them. The results obtained from the 10 sq. cm. 

 stamp between the main ribs show greater disparity between the weights of correspond- 

 ing pieces from the two halves of leaves than do the results with larger pieces in which 

 ribs were included. This I attribute to the irregular distribution of minor outstanding 

 veins, which cannot be avoided even in selecting very small areas. It also appears that 

 pieces 40 sq. cm. in area give better results than 20 sq. cm. pieces. In assimilation 

 experiments with Catalpa the smallest stamp was discarded, and the largest was used 

 wherever possible. 



VOL. LXXXII. — B. 2 M 



