66 BOTANICAL GAZETTE [JULY 
development when a single additional cell-division will determine the leap from 
one stage tothenext. The causes which incite cell-division may proceed gradually 
and continually but not until they have increased to a certain required degree 
will the additional cell-division take place, so that the effect is more or less dis- 
continuous, though the combination of causes may be continuous. 
It is also shown by MacLeop in the paper just i d by De BRUYKER,”® 
both working with Chrysanthemum carinatum, that the usual rule, according to 
which the earliest heads have the highest numbers, may be reversed by increasing 
the nutrition after the earliest heads are blocked out, thus fully demonstrating 
that the normal change which takes place from the beginning to the end of the 
flowering season is dependent upon the regular decline in the nutritional condi- 
tions. This seasonal periodicity is strongly emphasized in another paper by 
Dr BruYKER."? He finds that the number of flowers in the umbels of Primul 
elatior follows the same law of seasonal change as that followed by Centaurea, — 
Aster, and Chrysanthemum above mentioned, and now known to hold fora 
number of species. Specimens of Primula growing in dry places had a lowet 
mean number of flowers in the umbels than plants growing in moister places. 
The variation-curves for this character differ under different conditions and at 
different times, but always display modes on the Fibonacci numbers, 3, 5, 8,an¢ 
13. Evidence is given to show that these multimodal curves are not due to 
the presence of heterogeneity in the race, but are referable to the fact that develop- 
ment proceeds by more or less discontinuous stages, whose value in any particular 
instance depends upon the external and internal environment. : 
The complete dependence of the values of fluctuating characters upon the 
environment has also been well shown by KiEss'® in a comprehensive investi- 
gation of variation in the floral organs of Sedum spectabile. To avoid the posst — 
bility of dealing with mixed races, KiEBs has confined his studies to groups of 
individuals formed by taking cuttings from a single original plant. Growing 
these in several different habitats, he finds that for the stamens there isa different 
type of variation-curve in each habitat, which remains fairly constant so long ® — 
the habitat remains unchanged. Stamens of Sedum show the first of MacLeoo’s < 
three types of behavior, with 5 and ro as the favored classes. The various 
habitats produce curves ran ing from a monomodal curve with 5 as the mode 
and with very slight variability, through a bimodal curve with different relative 
values of modes § and 10, to monomodal curves with ro as the mode. Tt is show? E 
© De BruyKer, C., De gevoelige periode van den invloed der voeding OP br a 
aantal randbloemen van het eindhoofdje bij Chrysanthemum carinatum. 
rode Vlaamsch Nat. u. Gen. Cong,, Brugge, S. 1906. pp. 6. 
‘7, De polymorphe variatiecurve van het aantal bloemen bij Print 
elatior Jacq.; hare beteekenis en hare beinvloeding door uitwedige factoren. ‘aap 
e Vlaamsch Nat. u. Gen. Cong., Brugge, S. 1906. pp. 29. figs. 2. 
J "SK ers, G., Studien iiber Variation. Roux’s Arch. 24:29-113- A8% eo 
y. 1907. : 
