GROWTH AND MOVEMENT 419 



Primary meristem. — The formative regions in thallophytes arc 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 



