96 ELEMENTS OF PLANT ANATOMY. 
the stem, becomes meristematic. They grow, divide, and are 
filled with cell sap. This evaporates in such a manner as to 
cause a tension, which the new and weak walls are unable to 
bear ; they break and in this way a complete separation occurs. 
5. Comparative Anatomy of the Stem. 
Up to this point in the description of the stem, the entire 
development has been spoken of as coming from the meristem 
near the apex; this meristem les entirely above the protuber- 
ances described as the beginnings of leaves. These organs play 
an important part in the history of the growth of the stem, and 
it is owing to their connection with and influence over it, that 
its manner of growth is so different from that of the root. To 
make this clear, certain morphological relations must here be 
explained. It is usual to consider the methods of leaf arrange- 
ment as reducible to two, opposite and alternate ; the first, 
where two leaves appear to start from the same height on the 
stem and opposite each other ; the second, where the leaves all 
start from different heights on the stem, always with some reg- 
ular order of arrangement as regards their distance from each 
other measured on the circumference of the stem. 
According to the usual method of growth, the beginnings 
of the formation of the leaf organs follow each other so rapidly 
in the apical region that quite a number are formed before the 
oldest succeed in growing out to any considerable size, and 
while the longitudinal distance from one leaf to another is 
almost infinitesimally small. In case of winter buds the older 
leaves grow out as scales and extend entirely over the young 
stem tip, with its season’s growth of embryonic leaves and inter- 
nodes ready for development in the early spring time. The tip 
itself with its tender meristem tissue is protected by some of 
the inside leaves which entirely cover it. When the season for | 
renewed activity begins, the actual or visible extension in the 
