PINE ROOT 251 



partially disorganized : the cells of the endodermis will be dis- 

 torted by pressure from within : a brown corky tissue, varying in 

 thickness according to age, will be seen to have been derived 

 from the outermost layer of the pericycle. Centrally the dia- 

 metral plate will be seen as before, and the cambium on either side 

 of it will have formed secondary xylem internally, and secondary 

 phloem externally : note the primary phloem squeezed out of 

 shape by the increasing bulk of the internal tissues. 



Cut transverse sections of a still older root to inch in 

 diameter, and these will show the cortex thrown off entirely, and 

 the phloem, which is not clearly marked nor largely developed, 

 surrounded by the pericycle, with its peripheral layer of cork. 

 Internally the xylem will be seen, marked with annual rings as 

 in the stem, but centrally the diametral plate of primary xylem 

 may still be recognized. 



V. Cut median longitudinal sections of the apex of the 

 root of Pinus. This may be most easily done by cutting 

 longitudinal sections of the mature embryo in the seed. 

 Treat with potash or " eau de javelle " till they are 

 transparent, and mount in glycerine. Observe 



1. The central plerome-cylinder, recognized as in 

 the Sunflower and the Maize. It is rounded off at the 

 apex, and throughout is quite distinct from 



2. The periblem, which surrounds it: this is the 

 formative tissue of the cortex : outside this no true 

 epidermis is to be found, but at the apex is 



3. A root-cap, which is formed by the active divi- 

 sion of the cells of the periblem at the apex of the 

 root. 



Compare this arrangement of the apical meristem 

 with those types seen in the roots of Angiosperrns 

 (see p. 157, 183). 



