358 Blackman.- — The Compound Interest Law and Plant Growth . 
weight as H. uniflorns , would have to work at an efficiency of 0*2621 
(i. e. 26-21 per cent.) per day ; with this high economy of working the plant 
would double its weight in less than three days. For the highest production 
of vegetative material by the single plant two factors are necessary — large 
seeds, and a high economy in working represented by a large efficiency index. 
The importance of these two factors in breeding cereal crop plants 
should be borne in mind ; it may be possible to breed for a high efficiency 
index. In many crop plants the matter is of course complicated by the 
effect of crowding on the efficiency of the individual plant, a question which 
requires further analyses. 
The growth is naturally affected by external conditions, being higher 
when conditions are favourable, but even under the same conditions there 
are large variations in the economy of working of different plants, so the 
efficiency index is certainly to a large extent a characteristic of different 
species and varieties. It would be of great interest to determine to what 
these differences in efficiency are due. They maybe the result of differences 
in the rate of assimilation per unit area of leaf surface, of differences in the 
rate of respiration, of differences in the thickness of the leaves, or of 
differences in the distribution of material to leaves on the one hand and to 
the axis on the other. The larger the proportion of new material that the 
plant can utilize in leaf production the greater, other things being equal, 
should be its efficiency. 
It is clear, from Gressler’s and Hackenberg’s results with Helianthus 
and Cannabis , that the efficiency of the plant is greatest at first and then 
falls somewhat, but the fall is only slight until the formation of the inflor- 
escence, when there is a marked diminution in the efficency index. For 
Helianthus arboreus giganteus , for example, the £ substance- quotients ’ for 
successive weeks are 3-11, 3-49, 3-71, 3-06, 2-59, 3-03, 2-0, 1-5, 1-4, i-i, 1*3, 
1-3. The sudden fall from 3-03 to 2-0 is associated with the appearance 
of the inflorescence. 
The observations of Gressler, Hackenberg,&c., require repetition, for they 
worked with only a small number of plants and they give no idea of the 
experimental errors involved, though the differences in dry weight of the 
individual plants must have been considerable. It would be very valuable 
to have data for various agricultural and horticultural plants, showing the 
average efficiency index under various conditions. If the dry weight measure- 
ments were combined with a measurement of leaf area and leaf weight some 
insight could be obtained into the nature of the differences which exhibit 
themselves as differences in the efficiency index of different plants and as 
differences in the efficiency index of the same plant at different stages. 1 
1 If such observations were combined with an estimation of the carbon content of the root, stem, 
and leaf, together with some measure of the rate of respiration, the analysis could be carried still 
further. It is hoped to undertake this work with some agricultural plants. 
