CHAPTER IV. 
THE STEM. 
MORPHOLOGY. 
Just as the well-developed primary root may be regarded as a 
descending cone with apex below, so may the primary stem be 
looked upon as an ascending cone with apex above. This doubly- 
conical shape is characteristic of gymnosperms and dicotyledons 
(fig. 2). In monocotyledons, where there is no tap-root, the stem 
is typically in the shape of a cone with apex downwards, which is 
fastened in the soil by adventitious roots which proceed from it 
(fig. 4). 
The stem must be looked upon as the most important part of 
the plant, since it is the only one always present. Hair-structures 
are by no means constant, leaves may be absent, as in duckweed, 
where the little flat green expansions are stem structures, and 
lastly, roots, in rare cases, are non-existent, as, for example, in a 
kind of duckweed (Woljfia arrhiza), the body of which consists 
solely of a minute cellular green disc. 
A stem not only bears, as a rule, secondary stems or branches, 
which are similar to it in shape and structure, but also other 
1 This is a convenient place to remark that plants are classified into several 
large groups, which are again subdivided, and so on, till at last we reach 
species. These include individuals which differ only in slight features, such as 
size, and are evidently descended from a common ancestor. The subdivisions 
of plants in order of size are: group, division, class, subclass, series, cohort, 
order (or family), genus, species. Flowering Plants form a group with two 
divisions : (1.) Gymnosperms, including pine, fir, yew, juniper, larch, cypress, 
&e. ; (2.) Angiosperms, embracing most flowering plants, as, e.g., all those with 
coloured flowers. This division has two classes: (a.) Monocetyledons (the 
stmmaller division), taking in palm, arum, lily, tulip, onion, snowdrop, iris, 
sedges, grasses, &c. ; (b.) Dicotyledons, to which most British forms may be 
relegated, e.g., the majority of trees and shrubs, buttercup, wallflower, pink, 
poppy, Violet, waterlily, geranium, gorse, rose, daisy, dead nettle, primrose, 
&e., &c. 
Every plant has, for the sake of identification, a double scientific name, 
compounded of the names of its genus and species. For example, daisy is 
Bellis perennis. Bellis = genus, perennis = species. 
