VEGETABLE PHYSIOLOGY: COMPOUND ORGANS. 
27 
THE STEM. 
When the vital action of a seed is excited the tissue becomes 
developed in three directions; namely, upwards, downwards, 
and horizontally. That portion which elongates downwards is 
called the descending axis , or root; that which lengthens up¬ 
wards is the ascending axis , or stem; that which extends 
horizontally is called the medullary system ; and the part whence 
all these axes start is called the neck , or collum. The descend¬ 
ing axis is characterised by its avoidance of light, and by its 
penetrating the soil; from it are produced the fibres or feeding 
points: the ascending axis, on the contrary, is constantly 
striving to expose all its parts to the influence of light, and from 
it are developed, according to fixed laws, the leaves, and their 
appendages, as well as all the modifications in which these 
appear. As this mode of development is common to all plants, 
it follows that, during the earlier stages of growth at least, they 
all have these parts; and, therefore, when a plant is described 
as being rootless or stemless, such terms are not to be regarded 
as being physiologically correct. 
The ascending axis, or stem, demands our first consider¬ 
ation. The stems of plants are of two distinct kinds, and of 
each of these there are numerous modifications. 
Subterranean , or underground stems, are such as are very 
commonly called roots, from which, however, they essentially 
differ in their ascending tendency and origin, as well as by their 
structure. Of these, we have the cormus , which is the dilated 
base of the stem in some monocotyledonous plants; it is com¬ 
posed of cellular tissue traversed by bundles of vessels and 
woody fibre, and has the form of a flattened disk ; the Crocus, 
Colchicum, and Arum afford examples of this kind of stem : the 
tuber is an annual thickened stem, provided with buds, from 
which new plants are produced: in some varieties of potato, as, 
for example, that called the pineapple, the position of these buds 
indicates the real nature of the body on which they are produced ; 
whilst in others the excessive development of cellular tissue ren¬ 
ders them shapeless masses, and their origin is less perfectly por¬ 
trayed : the creeping stem is exemplified in the couch grass, the 
horizontal underground stems of which are vulgarly termed 
roots; it differs from the rhizoma only in being subterranean. 
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