NOTES AND MEMORANDA. 
335 
means of a slight pressure, the limb of a leaf, which remains attached 
to the living plant, and which only imprints hygrometrically those 
parts of its surface at which normally vapours are emitted. 
It is, moreover, to the vapours emitted, and not to the reactions of 
contact, that the imjn’essions thus produced are due, for they are 
equally formed through double folds of porous paper. 
With leaves of three morphological types they present the 
following characters : — 
1st. Leaves with stomata on the lower surface only. When these 
leaves have completed their development, the impression of their 
inferior surface, which appears distinctly from the first moments of 
the aj^plication of the blade, attains in a few minutes its maximum 
vigour ; and, during the short interval of time sufficient for its forma- 
tion, the upper surface does not make a sensible impression. As, 
however, it ultimately produces an impression on the hygrometric 
paper, there can be no doubt as to its exhaling power ; but it is 
always very weak, and may be j)ractically disregarded, in comparison 
with that of the inferior surface. 
In the impression formed by the latter, the venation is marked 
out, in white, on a more or less tinted ground which corresponds 
to the parencliymatous surfaces. These surfaces thus emit more 
aqueous vaj)Ours than those of the veins, although their cuticle is 
thicker and more waxy, and covers tissues less penetrated with 
moisture : their excess of emission can therefore only arise from the 
diffusion of the internal vapours through the orifices of their numerous 
stomata. It is especially in the exhalation of these leaves, taken at 
different phases of their development, that it is seen in what degree 
the activity of this function depends on the part which the stomata 
take in its accomplishment. 
So long as these little .organs are not formed, the two leaf faces 
exhale almost in the same manner ; but in j^roportion as they appear 
and multiply on the inferior face, the exhalation of this face increases 
rapidly, whilst that of the superior face decreases in consequence of 
the thickening of the cuticle and the strengthening of its waxy 
deposit. 
When the leaf is completely developed, the superior face plays only 
a very small part, which may generally be neglected, in the total 
exhalation, because it may be deprived of its evaporative property, by 
covering it with an impermeable varnish, without the leaf appearing 
to suffer. This same leaf, on the contrary, soon withers and falls, or 
rots in situ, when it is rendered impermeable on its inferior face. 
2nd. Leaves with stomata on both surfaces. In those of these 
leaves which belong to the group of dicotyledonous j)lants, the 
inferior face, having the stomata in larger number and evenly dis- 
tributed, gives impressions evenly shaded everywhere, on which the 
veins are marked out in white. The impression of the superior face, 
at once paler and unevenly tinted, shows thereby the relative rarity 
of the stomata and their uneven distribution, which it reproduces 
faithfully. 
In the leaves of monociffyledonous plants, the advantage, in 
