THE STEM. 24 
in their skins, reserves the most nutritive parts for the pigs. 
All the swollen underground stems just spoken of are especially 
characteristic of dry climates, for which they are well suited, as 
the condensed form offers comparatively little surface from which 
evaporation can take place. ; 
Modified Stems.—Hitherto we have had to do with typical 
undeniable stems, easily recognizable as such to the ordinary 
observer, except perhaps stem-tendrils. There are, however, 
numerous cases where stems are so modified, for the purpose of 
performing special functions, that they can only be recognized by 
homology. In other words, relative position and mode of develop- 
ment must be taken as criteria, and not the functions performed. 
This has already been spoken of on p. 7. Let us apply this to 
stem-tendrils. A direct continuation of an organ is evidently of 
the same nature as that organ. In the vine the youngest tendril 
is a continuation of the stem, and may therefore be regarded as 
part of it. This was previously the case with the youngest 
tendril but one, which, however, has been pushed on one side, its 
place being taken by a new stem borne in the axil of the youngest 
leaf. And so on for the next tendril. Thus, in any branch, the 
tendrils taken in succession, commencing with the oldest, have in 
turn occupied the end of that branch. We have therefore to deal 
with a sympodium (see p. 24). Another reason for not regarding 
the axis of a vine branch as a simple stem bearing branches in the 
usual way, is the fact that the tendrils do not grow from the axils 
of leaves. Examination of passion-flower tendrils shows that 
these do grow from leaf-axils, and we are therefore justified in 
considering them to be branches. Vine tendrils are further 
regarded as modified flower-stalks, because all possible gradations 
are found between the two. Stems may also bear branches modi- 
fied into spines and thorns for protective purposes. ‘These are 
known to be of stem nature from their axillary development and 
the presence of leaves upon many of them. It sometimes happens 
that stems undergo modification owing to the fact that the leaves 
are very small or absent. This means that some of the functions 
of the ordinary leaves have to be carried on by the stem, which 
in this case may be called a phylloclade. In cacti, for example, 
apart from the flowers, the leaves are reduced to minute spines, 
and the green stem assumes the most remarkable forms, globular, 
jointed, prismatic, &c., all of them very compact, and suited for 
dry climates (see above). In other cases, as asparagus and 
butcher’s broom (Ruscus), branches assume a flattened, leaf-like 
form, and are liable to be mistaken for leaves. Such phylloclades 
are termed cladophylls or cladodes. The stem of duckweed may 
perhaps best be placed in this category. 
