SPERM ATOPHYTA 



367 



Stem Tip. An embryonic region, a region of elongation, and a region 

 of maturation are present in a stem tip, but are more extensive and much 

 less clearly defined than in a root tip, overlapping to a consideral)le 

 extent. Immediately behind the apical meristem the leaf primordia 

 arise superficially as lateral outgrowths and develop in acropetal succes- 



FiG. 312. Cross section of a root of willow (Salix), showing a branch root pushing outward 

 through the cortex, X55. 



sion (Fig. 313). A meristem may arise in the axil of each leaf primordium 

 while it is still very small, giving rise to a lateral bud, or the meristem may 

 not appear until later. 



Generally the dermatogen is clearly recognizable, but often the line of 

 demarcation between the periblem and plerome is not. For this reason, 

 and because it is usually difficult or impossible to relate the origin of the 

 epidermis, cortex, and stele to distinct cell regions or "histogens," a 

 newer and more satisfactory concept of the structure and growth of the 

 stem apex is that it is made up of two distinct "growth zones." The 

 outer zone, or tunica, consists of one or more (up to four or five) superficial 

 layers of small uniform cells that divide anticlinally, so that each layer 

 remains distinct. Periclinal divisions occur only in connection with leaf 

 and bud formation. The inner zone, or corpus, comprises the central 

 tissue, the cells of w^hich are larger and divide in all planes and so are 



