146 BOTANY: PRINCIPLES AND PROBLEMS 



in thickness through the subsequent activity of a cambium 

 farther back along the root. 



The terminal growing-point at the apex of a stem resembles 

 in its essential features that described for the root, but the zones 

 in the growing region are not usually so clearly distinguishable 

 and their combined length is greater. The meristem proper is at 

 the very tip but a certain amount of cell division is going on 

 throughout the zone of growth, which here may extend over a 

 distance of several centimeters. 



Lateral Growing-point or Cambium. — The lateral growing- 

 point is somewhat more complicated than the terminal one and 

 its activities are often a little hard to visualize. The best 

 example of such a meristem is the cambium of the fibro-vascular 

 cylinder of root and stem, highly developed in all typical woody 

 plants. We have seen in our study of the stem (and except for 

 the absence of a pith the root is essentially similar) that the 

 fibro-vascular tissues are arranged in a cylinder, with a ring 

 of wood inside and a ring of bast outside. Between these two 

 rings is a very thin layer of tissue, in its resting period often only 

 one cell in width, which is formed of the same small, thin-walled 

 and richly protoplasmic cells which are characteristic of terminal 

 growing-points. This is the cambium, and by its activity the 

 fibro-vascular cylinder, and thus the whole stem, grows progres- 

 sively stouter. Unlike the one-sided terminal meristems, how- 

 ever, this lateral growing point adds to the tissues 07i both its 

 sides (Figs. 49 and 50). When the cambium is active and cell 

 division is taking place, the new cells which lie on the inner edge 

 of the cambium, next the wood, undergo a period of enlargement 

 and maturation and become the outermost wood cells; and the 

 new cells which lie on the outer edge of the cambium, next the 

 bast, similarly develop into the innermost bast cells; but a zone of 

 thin-walled "embryonic" tissue still remains between the two 

 (Fig. 73). Thus the cambium, never growing itself, continually 

 adds to the thickness of both its adjacent tissues. Just as the 

 terminal meristem is carried out by the growing root or stem tip, 

 so the cambium ring is carried farther and farther away from the 

 center of the stem by the growing wood ; and the bast, lying out- 

 side the cambium, is also carried out, not alone by this growth of 

 the wood but by the increase in its own thickness which has taken 

 place at its inner edge (Fig. 74). Cambial activity may perhaps 

 be crudely pictured by comparing it to the growth of a wall 



