64 GENEBAL BOTANY 



like the latter, have the power of active cell division, thus pro- 

 ducing new cells to replace those worn off at the apex by contact 

 with the particles of soil. There is no danger, therefore, that the 

 root will be deprived of its protective cap, even though it is con- 

 stantly losing its outer cells by death and abrasion from the soil. 



The meristem layer is one or two millimeters in length and is 

 composed of a mass of cells which fit into the upper, concave sur- 

 face of the rootcap. Its cells are more or less cuboidal in form, 

 with dense cytoplasm and large nuclei. The meristem layer is 

 therefore readily recognized in stained preparations, both by the 

 small size and regular form of its cells and by its darker color 

 (Fig. 32, a), due to the activity with which the dense protoplasts 

 of the meristem cells take up and retain artificial stains. The 

 nuclei are also large in proportion to the size of the cells and 

 often show clearly the heavily stained chromatin which makes 

 up the bulk of the nuclear protoplasm. Cell division is not so 

 frequently observed in the meristem cells as in those of the 

 elongating zone, although this process is one of the distinguish- 

 ing characteristics of the meristem. The reason for this seems 

 to be the extremely short period of time required by these cells 

 to complete the complicated process of cell and nuclear division. 



The elongating zone 1 occupies about four millimeters of the 

 length of the root immediately back of the meristem zone. 

 Its cells are distinguished from those of the meristem region 

 by their lighter color in stained preparations and by their 

 greater average length. The lighter color of its cells is due 

 to their less dense cytoplasm as compared with that of the 

 meristem cells, and to the gradual accumulation in their proto- 

 plasts of absorbed water in the form of vacuoles, which increase 

 in number and size toward the upper limit of the elongating 

 zone. We consequently find that the cells in the lower half 

 of the elongating zone resemble closely those of the meristem, 

 into which they graduate insensibly at the junction of the two 

 cell areas, while the cells of the upper half are longer and 

 lighter-colored, with large vacuoles. 



1 The upper part of the elongating zone, in which the cells are beginning to 

 differentiate into permanent tissue, is here designated as maturing. 



