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168 —— Structural and Physiological Botany. 
and inflorescences, the production of carbon dioxide by the 
inhalation of oxygen is very energetic; andysince the radia- 
tion of the heat produced by the oxidation is in these cases 
insignificant, from the surface being comparatively small 
or the parts protected by enveloping sheaths, very con- 
siderable elevations of temperature can be observed. At 
the time of the fertilisation of the spadices of Aroidese, an 
elevation of temperature amounting to as much as 10° C. 
has been detected ; and in the flowers of Cucurbitaceze and 
of Victoria regia similar elevations have been observed, 
though to a smaller extent. Respiration is a process dia- 
metrically opposite to the process of assimilation which takes “ 
place in the cells that contain chlorophyll, and which con- 
sists in the absorption and subsequent decomposition of 
carbon dioxide, although this latter has also been described 
under the same name, a use of terms as incorrect as if we 
were to speak of an animal inhaling its food. | 
The free zztrogen of the atmosphere enters the plant 
through the stomata, and also by the process of osmese, but 
_does not serve for nutrition ; for the purpose of nutrition this 
element must be presented to the plant in the form of nitric 
aeid or of an ammoniacal salt. Whether non-parasitic 
plants absorb nitrogen-also in the form of certain organic 
compounds is still undetermined. 
The only conceivable source of sz/shur is the sulphuric 
aeid of the sulphates present in the soil. It probably enters | 
the plant only in the form of calcium sulphate, which is de- 
composed by the oxalic acid formed in the plant itself; this 
being the source, on the one hand, of the calcium oxalate, 
which so commonly occurs in plants |in the form of raphides], - 
while on the other hand the sulphuric acid gives up its sulphur® 
in the production of protoplasm and generally of albuminoids. 
It is evident that the remaining nutrient substances can ~ 
only enter the plant by the process of diffusion, and in 
general in the form of soluble salts; but their relations to 
. the life of the plant are still obscure. With regard to zrom 
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