GROWTH AND MOVEMENT 
419 
Primary meristem. The formative regions in thallophytes are often 
rather indefinite, with a tendency in the higher forms to be restricted 
to the apex of the body. In the bryophytes they are found only at the 
apex, while in the vascular plants they persist commonly at both apex 
and base, i.e. at the tip of each axis and of each root. Here the active 
division of the formative cells and the differentiation of their progeny 
adds to the length of the body at one or both ends. There may be a 
single cell acting as the source of all, as in ferns, or a group of initials, 
as in seed plants (fig. 666). The repeated division of these initials and 
their progeny being the important feature, the formative tissue is des- 
ignated as meristem, and because this meristem persists from the earliest 
stage in the life history, it is the primary meristem. 
Secondary meristem. Regularly in certain regions and accidentally 
in others, tissues that have passed beyond the formative phase regain the 
power of division and exercise it for a longer or shorter time. Thus, in 
all plants whose xylem and phloem bundles show secondary thickening, 
a layer of cells between the two becomes a secondary meristem (cambium), 
and these initials may produce new cells on either face or both, which are 
gradually transformed into elements like their neighbors, while the in- 
itials continue to divide through the season, or function year after year. 
Again, a certain zone of the cortex or even the epidermis itself may 
resume active division, becoming a secondary meristem called the 
phellogen, whose offspring, the suberized periderm, constitutes a layer 
of cork protecting the surface (see fig. 539). Wounds, the presence of a 
parasite, or other stimuli may call again into active division almost any 
live cells, and the resulting tissues will cover the wound with a callus, or 
produce the deformity characteristic of the particular injury or parasite. 
Origin of branches. In the primary meristem of the stem the primor- 
dia of new organs are produced at the surface, the first indication of a 
new lateral branch, whether a shoot or a leaf, being a slight elevation 
of the surface, due to more rapid growth of cells at that point. This 
mode of origin is known as exogenous (fig. 666) and is characteristic of 
branches of the shoot axis. In the root, on the contrary, the first ap- 
pearance of a lateral branch is not at the surface, nor in the primary 
meristem, but at the limit of the stele or central cylinder (within the cor- 
tex), and among cells which have given over for a time active division and 
growth (fig. 667). The new branch must break through the cortex, since 
it is endogenous in origin; and this is characteristic of the root axis. 
Adventitious growing points, giving rise to new shoots, may appear in 
