340 —--' Canadian%Record of Science. 
pans a my TT 
excess of the requirements of growth, supply a most im- 
portant item of food for man. 
Various observations have been made to determine the 
amount of carbon dioxide which plants are capable of ap- 
propriating. The results obtained by . Boussingault are 
among the most instructive, from which we quote the fol- 
lowing :— 
Decomp. of CO, 
Area of leaf. per hour. 
Cherry-laurel.... 109 sq. ¢.m. 3:0) Ge: 
lei\aooGoacseeer 204 so ie ileal « 
Oakie ese ceeer Dm Os iLa@ «6 
Holly 5o0000 D050 yw & 6 1.8 ‘ 
Mistletoe... SQ e 2.0 a 
or for equal areas 
Cherry-laurel.... 100 “ “ 2400s 
JEN) cog005 HoOS00 OOD is ice O.o8)) © 
Oak tieiciesicucves sicis MOON cence 0.714 “ 
ISIN coonso650 MOOV Geert: 3.460 « 
Mistletoe ...... a LOO nee tee 2.000 “ 
In this connection it should also be noted that the pres- 
ence of carbon dioxide in the air, beyond a certain limit, 
causes it to exert a deleterious effect. This limit is of 
necessity variable, but observation has shown that in those 
plants which are most nearly allied to the coal plants, 
e.g., ferns, ten per cent. is fatal, while for the majority of 
plants, a much smaller quantity will produce the same result. 
The general process thus described, constitutes one of the 
leading features of the so-called digestive function, and as 
this takes place in the leaves (chiefly), they are usually de- 
signated the digestive organs. 
All the elements enumerated, except carbon, enter the 
roots which are specially adapted to the purpose of taking 
up food in a liquid form, and may therefore be designated 
the special organs of absorption. The power of roots in 
this respect, is nevertheless extremely limited with refer- 
ence to their total area, being confined to a narrow tract 
near the extreme tips, and is accomplished chiefly through 
the medium of root hairs. 
The fluid thus absorbed by the roots, and containing various 
