44 



PLANT BIOLOGY 



the hypocotyl of Indian corn which has grown about one 

 half inch. Make a longitudinal section. Note these points 

 (Fig. 47): (a) the tapering root-cap beyond the growing 

 point ; (b) the blunt end of the root proper and the rec- 

 tangular shape of the cells found there; (c) the group 

 of cells in the middle of the first layers beneath the root- 

 cap, this group is the growing 

 point; (d) study the slight differ- 

 ences in the tissues a short dis- 

 tance back of the growing point. 

 There are four regions : the central 

 cylinder, made up of several rows 

 of cells in the center (//); the en- 

 dodermis, (e) composed of a single 

 layer on each side which separates 

 the central cylinder from the bark ; 

 the cortex, or inner bark, (e) of sev- 

 eral layers outside the endodermis ; 

 and the epidermis, or outer layer of 

 bark on the outer edges (*/). Make 

 a drawing of the section. If a 

 series of the cross-sections of the 

 hypocotyl should be made and stud- 

 ied, beginning near the growing 

 point and going upward, it would 

 be found that these four tissues become more distinctly 

 marked, for at the tip the tissues have not yet assumed 

 their characteristic form. The central cylinder contains 

 the ducts and vessels which convey the sap. 



The Root-cap. Note the form of the root-cap shown in 

 the microscopic section drawn in Fig. 47. Growing cells, 

 and especially those which are forming tissue by sub- 

 dividing, are very delicate and are easily injured. The 



FIG. 47. GROWING POINT 

 OF ROOT OF INDIAN CORN. 



d, d, cells which will form the 

 epidermis; /, /, cells that 

 will form bark: e, t, endoder- 

 mis; //,cclU which will form 

 the axis cylinder; /', initial 

 group of cells, or growing 

 point proper; c, root-cap. 



