278 PRINCIPLES OF BOTANY, ETC, 
§ 272, | 
That plants transpire, has been said before, (§ 239), 
and that the leaves, as well as the stems and branches 
of trees, which are provided with the apertures be- 
fore described, (§ 236), serve these functions, ex- 
perience teaches us. Bonnet covered leaves with 
oil, and found that they grew black and decayed. 
Most ofthe philosophers, whohave made experiments 
on. this part of vegetable Physiology agree, that it is 
the upper surface of the leaf chiefly which performs 
the transpiration. However, it seems not yet de- 
cidedly proved, whether there is not in various plants 
some difference in that respect, and whether or not 
both surfaces sometimes equally transpire ? 
In young leaves we often see the transpired matter 
hang in form of small drops. <A young plant of. 
poppy, (Papaver somniferum), as well as young wheat, 
has, after cool nights, always a drop of moisture hang- 
ing on the points of its leaves, which disappears in 
day time, and in vain is looked for in the grown 
plant. Arum macrorhizon shews the same on its 
young leaves in our hot-houses. A new simple 
leaved species of the Mimosa from New Holland, 
has on the base of each leaf on its upper surface 
such drops. The Hibiscus abe/moschus has, on the 
under surface of its leaves, a great quantity of drops, 
§ 273. | bi 
Besides the moisture which the parts of vege- 
tables, especially their leaves transpire, they likewise 
give out gases. ‘This respiratory process, as it may 
A he 
