CHARACTERISTIC FORMS OF LEAVES AND SHOOTS. 21^ 
are leaves or parts of leaves which have become filiform, and possess the power 
of winding round slender bodies and thus of serving as climbing organs (as in 
Vma, Gloriosa, Smilax aspera, &c.). Leaf-spines are leaves which have developed 
into long, conical, pointed, woody bodies; they take the place of foliage-leaves 
{Berberis) or stipules {Xanthium spinosum, some species of Acacia). These two 
kinds of metamorphosis occur almost exclusively in Flowering plants (Angio- 
sperms), the morphological and physiological perfection of which, in comparison 
with Cryptogams and Gymnosperms, is especially due to the capability of their leaves 
to assume the most various forms. 
(2) Forms of Shoots. The axis of leaf-bearing shoots is, when sufficiently 
developed, usually columnar, with a cylindrical or prismatic surface. If the growth 
in length is very small in proportion to that in thickness, the short column forms 
a plate, as in the bulbs of Allium Cepa, and Isoeies ; if the growth in length is 
somewhat greater, with at the same time considerable increase of thickness, 
rounded or elongated masses are produced (as the tuber of the Potato and Arti- 
choke, the aerial stems of Mammillaria and Euphorbia meloformis) ; when the 
growth in length greatly preponderates we have stems, scapes, and filiform struc- 
tures of various kinds. Very commonly the same shoot shows differences of this 
kind in successive stages of its longitudinal growth ; thus the stem of the Onion, 
which is at first broad and tabular, afterwards rises as a high naked scape, the 
end of which in its turn remains short, and thus produces the umbellate in- 
florescence ; and in the same manner the thick tuber of the Potato is only the 
swollen end of a slender filiform shoot. Among the numerous deviations from 
the columnar form of the axis the conical is of peculiar interest. The conical 
stem is of two kinds; it may be slender at the base, increasing in thickness with 
further growth in length, so that each portion of the axis is thick in proportion 
to its youth, and the upright stem resembles a cone placed upon its point; the 
growing apex then lies on the surface which is turned uppermost, or rises above 
it as an upright cone. This form occurs in the stems of Tree-ferns, Palms, in 
Maize and in many Aroideae ; it depends on the absence of a secondary growth 
in thickness, while, with the age of the plant, the young tissue of the stem becomes 
constantly larger in circumference immediately beneath its apex ; when this in- 
crease ceases, the circumference of the later increment of length remains the 
same, and the inverted conical stem continues to grow in the form of a cylinder. 
The second form of conical stem is caused by a long-continued secondary growth 
in thickness together with the small circumference of the shoot at the growing point ; 
this occurs in Conifers and many dicotyledonous trees, the older stems of which 
are thick below but slender above, and thus resemble a slender cone placed on 
its base. 
The habit of a leafy axis or of a segment of one is usually in close relation 
to the number, size, and formation of its leaves. If the internodes are very short 
but the leaves small and numerous, the surface of that portion of the axis is nowhere 
exposed, and the leaves only are seen, as in species of Thuja and Cupressus, and 
some Mosses {Thuidiu?n) in such cases whole branch-systems frequently have the 
appearance of multipinnate leaves. If the closely packed leaves are large, they form 
a rosette enveloping the end of the stem, while the older parts of the stem are 
