K.—BOTANY 197 
geographical limits? This is a question of considerable scientific and 
economic importance, and concerns both the botanist and the forester. 
A complete survey of the form, habit, and growth of a tree within the 
limits of its natural range shows undoubtedly that each species can 
and does react to different environmental conditions, but opinion is by 
no means unanimous that these external conditions can bring about 
permanent change of an hereditary character. Late and early frosts are 
very troublesome and do much damage in the nursery, young regenera- 
tions and newly planted areas. Attempts have been made to obtain frost- 
resistant trees by collecting seed from the higher and colder elevations 
in the mountains, or from the northern and colder limits, but all such 
attempts have not yet solved the problem as far as frost-hardiness is 
concerned. A short consideration of the behaviour of young plants 
transferred from a colder to a warmer climate, and vice versa, may serve 
to bring out some points of interest in this connection. The four seasons 
vary in relative duration and climatic character according to latitude and 
elevation. This determines the length of the active period of vegetation. 
The critical seasons are spring and autumn. A certain amount of heat 
acting for a certain time is required to awaken the plant into vegetative 
activity, while the fall in temperature at the end of the vegetative season 
controls the rapidity and completeness of ripening and preparation for 
the resting season in winter. As regards the length of the active period 
of vegetation, the controlling factor seems to be the average temperature 
during that period. Further investigation concerning the commencement 
of vegetation and meteorological data are required, but as far as available 
information exists it would seem that each species of tree has an average 
temperature-constant which is necessary during its seasonal vegetative 
period. This period of average temperature is longer or shorter according 
as the tree is on itssouthern or northern limit. The effect of climate merely 
lengthens or shortens the period of vegetative activity, but the specific 
average constant of the tree is in no way altered. This has been called 
the vegetation therm by Prof. H. Mayr, who states that 14° C. is the 
constant for the larch, and probably also for the spruce. If such a figure 
could be fixed for all trees its value would be great, but this investigation 
necessitates further meteorological data and phenological observation. 
To return now to the question of the transference of a living tree from a 
warmer to a colder climate, or from a sheltered nursery to bare exposed 
planting ground. The chances are that if the transference takes place in 
autumn, the plant will suffer from early and winter frost. The plant 
has ripened off and prepared or attuned itself during the previous summer 
for the approaching winter conditions in general balance with the warmer 
climate, and it is not prepared for the earlier and more rigorous winter of 
the colder climate. On the other hand, if the transference takes place in 
spring after the winter resting period in its accustomed warmer climate, 
it has all the growing period in front of it, in which to adjust itself to the 
new conditions of the changed colder climate. This cannot be called 
acclimatisation, since the changes in the plant itself are not constitutional 
and hereditary. The tree will react to changed climatic conditions 
within its natural limits of distribution, but that is all. Ifa tree could be 
