118 ELEMENTS OF PLANT ANATOMY. 
characterized by great uniformity of anatomical structure, but 
this uniformity is found only in the lasting tissues. As regards 
the meristems at the point of vegetation there is greater com- 
plexity than at the corresponding point in the stem. The three 
primary meristems described as belonging to the stem are said 
to occur here and also a fourth is added in some instances, while 
in their manner of growth there are said to be, at least, six 
types. 
This complexity in the growing tip of the root arises from 
the peculiar nature and function of the root-cap. As the root 
never produces leaves, it lacks their protection over the soft and 
tender growing tissues of its apex, and this protection 1s sup- 
plied by the cap, which is a lasting tissue surrounding the meri- 
stems of the root tip. It is made up of parenchymatic, polyhedral 
cells. The inner rows of these cells, or those lying near the 
meristem, contain no intercellular spaces, but as new cells are 
added and the older rows pushed outward, their cells begin to 
separate at the corners and the tissue becomes less firm. This 
outer portion of the root-cap is subject to the continued friction 
of the soil through which it is pushed, and in this way its cells 
are gradually separated and thrown off from the rest of the root, 
their places being supphed by the cells lying next to them. 
This process is constantly going on, thus necessitating a con- 
tinual regeneration of the cap cells. So the meristem forming 
this organ is constantly active, although the cap itself is not 
extended in length as is the root proper. By this manner of 
erowth the meristematic cells of the root tip are protected from 
friction and also from too rapid transpiration. In aérial roots, 
the latter is the only danger to which they are exposed ; con- 
sequently their anatomical structure is somewhat different. 
The other lasting tissues of the root are grouped into three 
systems corresponding to those of the stem, epidermal, vascular, 
and ground systems. Like those of the stem, these also are 
supposed to arise from three primary meristems, dermatogen, 
