Meristems 75 



in culture promises to be enlightening. Here Steward and his collabora- 

 tors (1958) have done some significant work. They were able to grow 

 in culture dissociated phloem cells from the root of carrot. Some of these 

 produced multicellular masses. When a mass reached a certain size there 

 formed in it a sheath of cambium-like cells enclosing a nest of lignified 

 elements such as often is found in tissue cultures. In this spherical 

 nodule there developed a root meristem and then a shoot initial opposite 

 it. Thus an embryo-like structure was formed which was able to grow 

 into an entire normal carrot plant. Steward emphasizes the fact that the 

 change from random cell multiplication to organized development and 

 the formation of meristems comes only after the group of inner cells has 

 become enclosed by a wall of outer ones which cuts it off from direct 

 access to the coconut-milk medium outside and subjects these inner cells 

 to physical and presumably physiological restraints. Before this happens 

 they multiply irregularly and form simply an unorganized callus-like 

 mass. Such studies open up an important line of attack on the problem 

 of the origin of organized meristems. It will not be possible to understand 

 the role of meristems in development until we learn through experiments 

 like these how such an organized apical system comes into being. 



THE ROOT APEX 



The apical meristem of the root differs in several respects from that of 

 the shoot. It is relatively short, the elongating region of the root rarely 

 exceeding a millimeter in length. No lateral organs have their origin at 

 the apex, and thus there are no rhythmic changes here as in the shoot. 

 The lateral roots arise farther back, in the pericycle, and push out 

 through the cortex. The apical meristem produces not only the structures 

 of the root itself but, from its outer surface, the root cap, or calyptra. 

 Root meristems received much attention in the early work of Eriksson 

 (1878), Flahault (1878), Holle (1876), Janczewski (1874), and van 

 Tieghem and Douliot (1888). 



In those lower vascular plants where shoot growth is centered in an 

 apical cell, the root grows in the same way (Fig. 4-2). In the higher 

 plants, however, although there is a meristem which is in many respects 

 like that of the shoot, there is less uniformity in its organization. In 

 most roots there are seen well-marked layers and to these some workers 

 have applied Hanstein's terminology. The direct origin of particular tis- 

 sues from particular layers is not uniform, and the same objections to 

 regarding the layers as histogenetic ones may be made as for the shoot. 

 The presence of the root cap prevents smooth and continuous layering 

 over the root apex, and this is probably the reason why periclinal chi- 

 meras are not found in roots. 



