INFLUENCE OF CLIMATE ON PLANTS. 
59 
this member, the new alburnum will be formed on the outside ; but if placed on 
the exterior side, the new deposite will be invariably formed within it ; and just 
where it would have been had no paper been inserted." 
These remarks are very striking ; but if any one start an objection of the 
infinite minuteness of parts, he maybe reminded of the small dust of the fern-tribe, 
every particle of which contains the rudiments of a plant, and many species assume 
a gigantic growth. Be the ftict what it may, it certainly appears most philo- 
sophical to conclude that the annual layers are strictly develoijments^ and not new 
creations ; and that the enlargement of organs is a process of nutrition, not of 
abstract production. 
INFLUENCE OF CLIMATE ON PLANTS. 
SOLAR HEAT. 
{Continued from "page 37.) 
From the general principles already premised respecting the agency of heat on 
vegetation, we proceed to specify a few of its more particular effects ; interspersing 
our remarks with plain and practical hints relative to the instances in which their 
knowledge is useful in artificial cultivation. They may be comprised in three 
principal divisions : mz. the degree in which the geographical diversity of plants 
is afi*ected by heat, the influence it exercises upon their functions, and the extent 
to which adventitious circumstances, such as elevated tracts, valleys, forests, exten- 
sive collections of water, &c., modify and regulate its diffusion and intensity. An 
Inquiry into these several subjects will be, we conceive, not only interesting in a 
physiological point of view, but afford ample data whereon to attempt a system of 
acclimatizing plants, and supply authoritative bases for any future observations on 
the regulation of artificial temperature. 
That heat is eminently instrumental in the geographical distribution of plants, 
by assigning and restricting those of a certain habit and character to special dis- 
tricts, is indisputable. Almost every climate on the surface of our globe has a 
vegetation more or less peculiar to itself ; and although various circumstances 
concur to effect this limitation, temperature is unquestionably the chief. Indeed, 
not only does heat, by its presence in different degrees, determine the localization 
and range of plants of particular and kindred habit, but it is exceedingly plau- 
sible and probable that the modification of heat has tended much to create the 
diversity of habit in different zones. Thus, the vegetation of each zone exhibits 
its ow^n peculiarities and characteristics ; that is, we meet with plants of a certain 
habit in their appropriate region, not only because the mean temperature is pro- 
pitious, but to a certain extent, and in the lapse of ages, it is extremely likely that 
the temperature has conduced to the causation of the distinguishing habit existing 
in such clime. 
In endeavouring to ascertain the influence which heat possesses and exercises 
