Blackman . — The Compound Interest Law and Plant Growth . 357 
of assimilation of 2 per cent, would in this case increase the weight at the 
end of 32 days by about 20 per cent. 
A marked difference in the rate of interest (r) is exhibited by different 
plants. The table given shows that in some species of Helianthus it may 
reach 17*6 per cent, per day, while in H. cucumerifolius nanus it is only 
10*42 per cent, per day. In some results obtained by Stefanowska 1 with 
Maize in water culture the plants increased their fresh weight 27-5 times 
in 45 days ; the rate of interest was therefore only 7-45 per cent, per day. 
Obviously some plants can work with far greater economy than others. 
Thus for every 100 grm. of dry material already present H. macrophyllus 
giganteus can produce new material at the average rate of 10-4 grm. per 
day ; calculating from Hackenberg’s results, we find that Cannabis gigantea 
may work at a rate of 13-1 per cent, per day for a short time, and (from 
Kiltz) that Nicotiana Tabacum may work at the rate of 20*5 per cent, per 
day. Zea Mais , on the other hand, as stated above, under some conditions 
works at the average rate of only 7-45 per cent, per day. 
The rate of interest (r of the equation) is clearly a very important 
physiological constant. It represents the efficiency of the plant as a producer 
of new material , and gives a measure of the plant’s economy in working. 
The rate of interest, r, may be termed the efficiency index of dry weight 
production, since not only does it indicate the plant’s growth efficiency as 
measured byincrease of dry material, but it also appears as an exponential 
term in the equation which expresses the relation between the initial dry 
weight, the final dry weight, and the period of growth. It may also be 
termed the 'economy constant’ of the plant ; it is of course comparable 
to the velocity constant of a chemical reaction. 
It is suggested that in all water cultures, pot experiments, and similar 
experiments where dry weights are determined after a period of growth, the 
efficiency index should be calculated from the seed weight and the final weight 
attained, so that a measure of the plant’s average economy of working may be 
obtained. 2 Such a calculation will show how far a large final weight is 
determined by a large initial weight or by a high efficiency index. 
A glance at the table given above shows that the ‘ dwarfness ’ of 
Helianthus cucumerifolius nanus is due not only to the very small seed 
weight but also to the comparatively low efficiency of the plant, the efficiency 
index being only 0*1042 (or 10-42 per cent.) per day. This form of 
Helianthus is handicapped by a seed i/300th of the weight of that of 
H. uniflorus giganteus , so that even if it had the same efficiency it could 
only attain 1/300 of the final weight of the latter. Other things being equal, 
a small seed is a permanent handicap to a plant in the production of 
material. H. cucumerifolius nanus , in order to attain after 37 days the same 
1 Comptes rendus de l’Acad. des Sciences, cxxxviii, 304-6, 1904. 
2 Where root parts are not available, the efficiency index can be given for the aerial parts only. 
The seeds should, where possible, be weighed without the testa. 
