THE STEM. 23 
as in the case of the root, may be regarded as slender cones 
attached to the side of the primary one. Here, too, their suc- 
cession is regular and acropetal. Adventitious stems may also 
be formed, and this is the nature of most of the slender twigs 
which often sprout from old tree stumps. 
The majority of stems are aérial, but water stems, partly or 
entirely submerged, are naturally present in marsh and water 
plants. <Aérial stems are sometimes parasitic, as in clover dodder, 
which possesses suckers on its stem that become intimately united 
with the stem of the clover. It is commonly imagined that all 
underground structures are roots. This, however, is a mistake, 
since a great many stems are more or less subterranean. 
The external form of a stem is influenced by a number of 
factors. The most obvious is that of size and duration. Upon 
this depends the old and somewhat vague classification into herbs, 
shrubs, and trees, as well as the division already mentioned into 
annuals, biennials, and perennials. 
Herbs are plants, usually of no great size, in which either the 
whole plant dies after a year’s growth (annual herbs), or else the 
subaérial portions die away every year (biennial and perennial 
herbs). The stem is not at all or scarcely woody. Shrubs are 
perennials, mostly less than twenty feet high, with woody stems 
that branch from or near the ground. Trees are perennials, 
generally more than twenty feet high, with a distinct woody 
primary stem or trunk. There are, however, no sharp boundaries 
between these divisions. Herbs pass into shrubs, the intermediate 
small woody perennials being called wndershrubs ; and shrubs, 
again, are connected by all possible gradations with trees. The 
names herb, &c., refer to the shape of the overground parts of the 
plants taken as a whole. The form of the individual stem is, m 
the majority of cases, cylindrical and solid. Some herbs, how- 
ever, possess square or triangular stems, while ribbed and flattened 
forms are not uncommon. It may happen that the last kind are 
winged, or produced at the sides into thin green expansions. 
Hollow or jfistular stems are often found among herbs, growth in 
thickness having exceeded increase in substance, the result being 
that the internal tissues have been ruptured. ‘This is very typi- 
cally seen in grasses, where the tubular internodes are separated 
by swollen nodes. Bamboo and any kind of straw will serve as 
examples. ‘These stems are called culms. Not only may the size 
of the whole stem system be considered, but also the size of the 
individual parts as regards length and thickness. The maximum 
length is attained in the trunks of trees, where, as in all long 
stems, the internodes are well developed. Great development of 
the internodes may cause the stem to be thin and weak, as in 
