VEGETABLE PHYSIOLOGY. -THE LEAVES OF PLANTS. 179 
is absent, and tlie leaf is then said to be sessile. In simple 
leaves the petiole is continuous with the axis of the lamina, 
from which it never separates ; but in truly compound leaves 
it is articulated with each stalklet, so that when the leaf perishes 
it separates into as many parts as there are leaflets. Hence, if 
an apparently simple leaf is articulated with its petiole, as in 
the orange, such a leaf is not to be regarded as a simple one, 
but as the terminal leaflet of a pinnated compound leaf, the 
lateral leaflets of which are not developed. 
The petiole, though occasionally absent, as in some few 
plants with sessile leaves, is sometimes found to supply the 
place of leaves, and in this character is called phyllodia : the 
so-called leaves of a great number of Acacias are of this nature, 
the phyllodia being developed so as to assume the appearance, 
as well as exercise the functions, of true leaves; they may be 
known from the latter, by their surfaces being alike, by pre¬ 
senting their margins, not their surfaces, to the earth and 
heavens, and by their being straight-nerved, which, as they 
occur among dicotyledonous plants having reticulated leaves, is 
alone sufficient to distinguish them. 
In some cases the petioles become hardened, and they then 
assume the appearance of spines , as in the common barberry ; 
the spiny petioles of some leguminous plants are of the same 
nature. In some cases the petiole is elongated, and endowed 
with the power of twisting around any small body near it, as in 
the pea; in other cases it passes in the form of a twisted mid¬ 
rib beyond the apex, as in Gloriosa. The pitchers of Nepenthes 
and Sarracenia are supposed to be modified petioles. 
The usual posture of leaves on the plant is with their sur¬ 
faces presented to the sky and earth, but this is not universally 
the case : the effect of imbrication in some instances, and of 
deflexion in others, is to bring the upper or lower surface 
respectively in a line nearly parallel with the stem ; and in some 
cases, especially in such plants as bear phyllodia, the margins 
are presented to the sky and earth, generally, though not 
always, by means of the twisting of the petiole. 
The position of leaves on the stem varies considerably, al¬ 
though their disposition seems to be regulated by the same fixed 
laws which are so admirably displayed in all the works of cre¬ 
ation. Sometimes leaves are placed opposite, and sometimes 
alternate ; but it seems to be probable that the normal position 
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