92 
THE PRINCIPLES OF BOTANY. 
ing to species ; some being single, as in tlie ivy, some in pairs 
opposite each other, as Laiirustinus, others in whorls of 
several springing, as it were, from a collar all round the 
stem, as the Woodruff. 
Again, the leaves at the base of a plant will be sometimes 
different from those of the stem, hence, then, the radical and 
cauline leaves will have to be distinctly discriminated. 
The Morphology of Leaves is perhaps amongst the most 
interesting of botanical subjects, and we shall, therefore, now 
only glance at a few of these. 
If the student will examine the garden pea while growing 
he will see that it is a climbing plant, which keeps itself in 
an upright position by twisting a sei’ies of wire-like tendrils 
around the twigs of the supporting sticks. JNow, if a perfect 
leaf be first looked at, it will be seen to be a pinnate leaf, 
consisting of perhaps two pairs of lateral leaflets, with some¬ 
times a terminal leaflet of a like kind, but more frequently 
the pagina of this terminal leaflet is wanting, as the nerve 
has elongated into a tendril. So, again, it will be found 
that there will be examples of the two upper leaflets being in 
like manner metamorphosed into tendrils. Nay more, all the 
leaflets will sometimes be so changed, and thus a new, and 
apparently complicated and distinct, organ for a special pur¬ 
pose has been constructed out of the simple leaf elements. 
But the most curious examples of leaf morphology may be 
seen in the tropical houses of our best plant collections in 
the pitchers of the Nepenthes. In this plant the lower part 
of the petiole is winged with pagina, and this part, indeed, 
performs the leaf function. This kind of leaf elongates 
upwards into a strong wiry stem, by which the pitcher is sup¬ 
ported, this pitcher being another expansion of leaf matter, 
but here with the edges cemented at their margins—hence 
the hollowed pitcher. Here, then, the lower part of the 
petiole has been expanded into a leaf, the middle part being 
a really supporting stem. It, however, supports a pitcher, 
while the true leaf must, after all, be sought in the curiously- 
formed lid by which the mouth of the pitcher is closed. 
Now, strange as all this may seem, the initiative of these 
metamorphoses may be made out from the leaves of orange 
and lemon, which, to the common observer, appears to be 
simply an ovato-lanceolate leaf, while in reality it is highly 
complicated in structure, consisting of a flattened blade; it 
is true, but this is articulated to a petiole, which is dilated or 
winged after the same manner as that of the base of the leaf 
of the pitcher plant. Viewed in this way these leaves will be 
found to be capable of endless morphological changes, and 
