10 OUTLINE OF STRUCTURAL BOTANY 
The stems of flowering plants may be divided into two classes 
upon which divisions are in large measure founded the two great 
groups of plants known as Hndogenous and Exogenous plants. 
In the first of these groups, endogens, including the grasses, 
palms, liliaceous and many other plants, the accretion of growth 
is from within the stem. This latter does not become thicker as it 
extends in height. This statement will not appear correct when 
one remembers that the stalk of Indian corn is nearly or quite an 
inch in diameter at maturity, while in its early stage it has only 
a fraction of that diameter. This is not because the diameter of 
the first sprout has materially increased, but that successive joints 
or nodes have arisen from the root, each of greater diameter than 
the one preceding it. This form of growth may be observed in the 
grasses, rushes and all of the plants known as monocotyledonous 
plants. On the other hand, in another great group of plants, the 
exogens, the growth of the stem is from within outward. The 
stem of the ordinary tree of temperate climates increases in diame- 
ter in proportion to its growth, the accretion being made externally. 
This is true of most of the species of herbs growing in the same 
climate. In the stem of the endogen there is no distinction of 
bark, wood and pith, but in the stem of the exogen this distinction 
is clear, at least in the very young plant. The distinction of the 
bark from the wood is evident throughout the growth of the plant. 
Upon the character and size of the stem depend the division of 
plants into herbs, shrubs and trees. 
Plants, the stems of which do not become woody and which die 
down to the ground at the close of the season, or after flowering, 
are known as herbs, while those the stems of which become woody 
or which are persistent from year to year, are, if of small size 
when of mature growth, shrubs, but if the plant reaches or exceeds 
about twenty feet when fully grown it is a tree. 
While in the greatest number of species of plants the stem rises 
from the ground, standing erect or nearly so by its own strength, 
* upright stems, there are others 
Be Tis which have too little strength 
*: thus to rise independently. 
Their length is usually out of 
= proportion to their diameter 
to enable them to stand up- 
Pia. 12 right without assistance. Some 
of these are procumbent stems which creep along the ground like 
the weak stem of the common blue veronica (Fig. 12), its head 
