88 Franklin Kidd , Cyril West and G. B. Briggs . 
When cell w^lls are formed later a hexad of pollen grains is 
frequently produced, four large ones containing seven chromosomes 
and two small containing one or two chromosomes each. It is 
doubtful if any of these pollen grains are functional—regular 
meiosis takes place occasionally—but it is interesting that a pollen 
grain with only one chromosome, though diminutive, can still form 
its exine covering in an apparently normal manner. And such 
irregularities always make it possible that an extra chromosome 
may find its way into one of the daughter nuclei, as in the origin of 
(E. lata . 
WHAT IS THE SIGNIFICANCE OF THE EFFICIENCY 
INDEX OF PLANT GROWTH ? 
By Franklin Kidd, Cyril West and G. E. Briggs. 
[With Two Figures in the Text] 
I N a recent paper under the title of The Compound Interest 
Law and Plant Growth, V. H. Blackman (1) has suggested 
that the dry-weight of an annual plant increases at continuous 
compound interest, expressed mathematically by the formula 
W l =W 0 e rt where is the dry-weight of the plant at the end 
of the time t, W 0 the initial seedling or seed weight, the rate of 
interest and e the base of the natural logarithms. 
Subsequently a paper by Brenchley (2) has appeared in which 
the writer has applied this formula. Although the conception 
of plant growth as a process similar to that of money increasing at 
compound interest is a useful one, and indeed was put forward by 
two of the present writers in a previous communication (5), we think 
it is already clear that a more complete analysis of existing data on 
plant growth does not warrant so rigid an application of the 
conception as that made by Blackman and adopted by Brenchley. 
The present writers have just carried out a detailed analysis 
of the carefully recorded growth data of Kreusler and his 
