'ipy -VEGETABLE PHYSIOLOGY. 87 W 
tJ ill a much shorter space of time in a climate which has a hotter summer ; but, on the other hand, 
manj' plants which only occupy a short space of time in theu- growth in a climate where the summer 
is hotter, cannot be made to spread theii' season of growth over a longer period in om' cooler climate, 
because they wiU not flom-ish below a certain temperature, and that begins too late and ends too soon 
in our climate. Thus, the Vine and the Indian Corn flourish on the Continent, in places which have a 
far more severe winter than we have ; but the cause of tins is, that the siimmer is very much hotter 
at the time these jiarticular plants require much heat. 
Practically, these facts are of much importance in regard to the successful treatment of perennial 
exotics ; since these may be injui'ed by being allowed to vegetate weakly during the period in which, 
in theii' native climate, the severe cold would have arrested their growth, and be thus weakened, so as 
to interfere with their successful forming into flower at their proper season. It is of great importance, 
therefore, to studj' the pecuHarities of the native climates of the plants we cultivate, since it is only 
by an intelligent imitation of these, that we can hope for certain and safe rules for the management of 
the plants, during their various periods of growth. 
The effects of light upon vegetation are perhaps the most important of all the physical phenomena 
we have to study in plants ; but wide as is the field for observation here, we have as yet advanced 
but a little Way into the subject. The now well-known Daguerreotype process gives us a means of 
conveying an idea of the manner in ■^'ihieh light may aflect bodies ; a still more simple one is its 
blackening action upon any organic substance, such as paper, which has been touched with nitrate of 
silver (lunar caustic). In these we see the effect of impalpable, intangible force, producing a chemical 
change, made known to us by a change of colour. By just such a change of colour does it announce 
to us its action in plants ; in proportion to the force with which light exerts chemical action upon 
plants, do we see their green coloui- become deeper ; while, if we withliold this influence, as in the 
blanching of various kitchen-herbs, we find that the green is not produced. 
To go into a detailed account of the present state of oui- knowledge in this department of the 
subject, might involve us in questions which are beyond the purpose of the present sketches, since 
recent discoveries have shown that it is not perfect white light simply acting upon the contents of 
the tissues that we have to study, but that we must inquii-e into the especial efiects of the different 
component rays which the prism reveals to us, if we would thoroughly understand the mode of 
natm-e's operations here. The ray of common light is a compound of the various coloui'ed rays which 
are displayed in the prismatic spectrum, in the rainbow, and these act upon plants, not only jointly, 
but individually ; thus the yellow part of the light exerts a different action fi'om the blue, &o. 
Without going fui'ther into this part of the question, I wUl merely caution horticidtm-ists that 
deductions from the facts as yet before us are scarcely advisable, except for experiment ; and although 
certain coloured lights may favom- the development of plants dui-ing particular periods, yet as a general 
rule, and in houses intended to keep plants dm'iiig their whole periods of growth, it is clear that the 
perfect white light, which natm-e gives them in all climates, must be the best, and that when this 
may be too strong, shading must be used which wiU lessen the whole light, and not take away 
merely certain rays, as will be done by any coloured glass. Blue light has been said to favour germi- 
nation, yellow Ught, flowering, and if this be proved, a blue house might be a useful place for raising- 
seedlings, a yellow one for flowering- plants ; but the worst of such expedients is, that a portion of light 
is absolutely lost in passing thi'ough these eoloui-ed media, and in any case they are rmnatm-al, and 
therefore to be regarded with suspicion. 
With regard to the direct universal action of light upon plants, it is I'equisite to know something 
of the internal structure of plants to understand the processes clearly, and I shall therefore have to 
refer to it again hereafter. For the present, it will suifice to say that, by a peculiar power residing 
in it, light produces a chemical change in the fluids contained within the cells of plants, and, since 
this is attended with a separation of certain materials which are given off in the form of gas, as 
carbonic acid is given off by animals in their breathing, it has generally been called the respiration of 
plants. Some authors question this mode of -^dewing the process, and regard it as a digestive opera- 
tion ; it seems, however, so far as we know at present, to be a true respu-atory process, and one 
similar in character, but directly opposite in its effects to that of animals. Animals, by means of 
respiration get rid of superfluous carbon, but the process is here combined with a most important 
function, the maintenance of the animal heat, which is effected by the carbon being oxidised or burnt 
by the oxygen of the air, in a manner which, however sm-prising it may seem to those who learn it for 
the first time, exactly resembles the more active burning of charcoal, or other carbonaceous matter, 
<K where the greater heat is accompanied vs-ith flame. In vegetables it is oxygen that is given off (which 
jRj is consumed by being combined with the cai-bon in animals), while the carbon remains fixed in the 
