a 
Relation of Climate to Vegetation. | La 
for any species, we will find them conforming to those lines 
of equal temperature which Humboldt designated isother- 
mal. It is therefore easy to understand that the mi- 
gration of plants is accomplished with the greatest difficulty 
in direction of latitude, but that it becomes a comparatively 
simple matter for them to extend in direction of longitude, 
Itis a recognition of these laws which should guide us 
whenever we desire to introduce exotic plants for the 
adornment of our grounds, or to add new resources to our 
food or forest supply. 
These laws are also expressed in the germination and 
growth of plants. It is a well recognized law of vegetable 
physiology, that while a certain temperature is essential to 
the germination of seeds, the requisite degree of heat is 
not the same for all plants, and in fact often differs widely. 
The same may be regarded as true of growth after germina- 
tion. We may therefore indicate the lowest temperature at 
which germination can begin, and also the best temper- 
ature for growth as follows :— 
Germ. Best Growth. 
Wheat and barley ...... -..c0s ses « 41° F. 83.6° 
ABO Me tee oleletiietele.et sis'oisievaverstevanarsiets AS De 79.9° 
WUUS PM tes esses ae sy eek iaeionelaierehni tals) siete els 48 .0° 92.6° 
ESSN a oe onic oo! Wee eo e ook olaicr o; ava iaiajeley ovate s 48.0° 92 .6° 
Squash ..-..- ie S epeiara co eats atta) ciety eet ee 56.6° 92.6° 
Such facts as these are significant and could readily be 
made to apply to all plants. 
A very interesting, and in some respects important effect 
of climate upon vegetation, and more especially upon the 
arboreal forms, is to be seen in the correspondence between 
climatic periodicity, and periodicity in growth with corres- 
ponding modification of structure, 
An examination in Gross section, of any of our common 
trees such as the maple or elm, will show that the woody 
trunk is built up of a series of concentric rings, and if we 
follow the growth of such a tree from year to year, it will 
appear that these rings coincide more or less closely with 
the alternation of seasons, one ring for each year, in conse- 
