138 THE BIOLOGY OF FLOWERING PLANTS 



before assimilation can attain its maximum rate ; the 

 presence of chlorophyll alone is insufficient. At the same 

 time the degree of development of the reactive surface 

 affects both stages of the process, for a more extensive surface 

 means a greater number of molecules activated by light, and 

 it also means a more intense interaction of the activated 

 molecules. 



Similar relations were found by Willstatter in ageing 

 leaves. In leaves which turn yellow, the decrease in the 

 chlorophyll is accompanied by a decrease in the rate of 

 assimilation. The assimilation numbers tend to rise for a 

 time, and then to fall to a value lower than that of the mature 

 leaf ; that is to say, the smaller amount of chlorophyll at 

 the end is less advantageously used ; or, to put it otherwise, 

 the protoplasmic factor is failing. This is still more 

 marked in leaves which remain full green even after autumn 

 frosts. In these, although the chlorophyll content may be 

 quite high, assimilation may almost or altogether cease. 

 Yet such leaves, if kept in a warm moist atmosphere for a 

 few hours, recuperate. Willstatter found, for bright green 

 leaves of Ampelopsis Veitchii gathered on the 17th of 

 November, an assimilation of o"oo6 grm, of carbon dioxide 

 per hour ; after a day at 25° C. they assimilated six times as 

 vigorously ; the assimilation numbers were less than 0*8 

 and 4" 6 respectively. 



Of interest, too, are the results obtained with normal 

 and golden varieties of the same tree ; thus, " aurea " 

 varieties of oak, elder, and elm possess one-thirteenth, 

 one-thirtieth, and one-tenth respectively the amount of 

 chlorophyll of the normal green varieties, yet their assimi- 

 latory activity is one-half, two-thirds, and equal to, that 

 of the green varieties. The assimilation numbers corre- 

 sponding are enormous, i.e 55, 113, and 77, compared with 

 7'8, 6'4, and 8*4 for the green leaves. Here we have plants 

 with only a trace of the normal amount of pigment showing 

 assimilating capacities approaching that of the full green 

 leaf ; in conditions of intense illumination they are probably 

 as efficient as the normal varieties, and indeed one may 



