95 
functional activities at a lower average temperature, and this 
process of adaption has in some cases gone so far as to render 
certain plants able to exist above the perpetual snow line. 
The same lowering of the optimum seems to be still going 
on in plants; for the average optimum for plants in general 
is about 80° C., which is considerably higher than the average 
temperature to which vegetation as a whole is exposed. Hence 
in the tropics as regards temperature at any rate somewhat 
more archaic conditions are preserved than in the temperate 
regions, where the different functional activities have undergone 
more or less marked adaption to the colder habitat. Thus the 
function of CO,-assimilation ceases in tropical plants at a 
temperature 5 to 10° C. higher than that necessary to stop 
assimilation in the plants of temperate regions '). 
It is frequently stated that the existence of a certain pe- 
Niodicity or alternation of periods of greater and less activity 
is a fundamental necessity for continued life, and a consideration 
of European plants alone might have led us to the conclusion 
that a winter resting period was essential for the maintenance 
of plant life in general. In certain cases a periodicity bai de- 
velopment originally induced by regularly recurrent variations 
in the external conditions seems to have become permanently 
fixed, as for example in obligate heteroecious parasites and a 
Ptophytes, as well as in all plants exhibiting definite alternation 
of generations, but even here it seems that under appropriate 
Conditions the development may be direct, one stage or the other 
ing eliminated. It is true that the life of a species from 1- 
dividual to individual is periodic, but this applies only to those 
Pants in which special reproductive cells have been developed, 
di- 
and & bacterium might continue under constant optimal ia 
tlons to exhibit an unchanged and unweakened vital activity . 
an indefinite length of time, so long as no spores were sine of 
_ Periodicity is not therefore an essential characteristic 
Ying matter but is always induced by recurre 
1 pee lea 
!) Assim. Inhib. Journ. of Linn. Soc. XXXI, p- 399- 
nt changes in 
