THE ROOT. 
51 
the result being a longitudinal contraction of the root 
at this point. In this way the plant is fastened more 
securely to the earth, and at the end of the season’s 
growth the apical buds of plants, with upright rhi- 
zomes, as of Veratrum viride, Dracontium, etc., are 
drawn into the earth and thus protected during the 
winter. 
Abnormal Structures of Roots. — It is frequently diffi- 
cult to recognize the type-structure of dicotyledonous 
roots, owing to the development of anomalous and 
abnormal secondary structures. Sclerenchymatous 
fibers, while present in glycyrrhiza and althaea, are not 
infrequently wanting. Wood fibers may be sparingly 
developed, as in belladonna, or even wanting, as in 
gentian. In other cases the medullary rays are abnor- 
mal, being replaced in calumba by wood parenchyma, 
and in ipecac and taraxacum by sclerenchymatous cells 
In asclepias and calumba a layer of stone cells occurs 
near the periphery ; in gentian, sieve cells develop in 
the xylem ; in senega the xylem is not uniformly 
developed, and in still other cases, as in jalap, pareira 
and phytolacca, successive cambiums develop, produc- 
ing a concentric series of fibro vascular bundles. 
II. THE STEM. 
The stem, or ascending axis of the plant, grows in a 
direction opposite to that of the root, seeking the light 
and air. The tendency of the stem to grow upward is 
characteristic of the majority of plants, and is spoken 
of as negative geotropism. The growing point of the stem 
is at the apex, and it is protected by the embryonic 
leaves. 
Stems are further characterized by bearing leaves, or 
modifications of them. The leaves occur at regular 
intervals in the same species, and that portion of the 
