ANATOMY OF TISSUES. 5 M3 
details, but a general description of the beginning of the bundle 
cylinder and its subsequent development may be given as follows. 
The origin of the root bundle differs in several respects from 
‘that of the stem. This may be illustrated by a cross-section 
near the root tip where there are four xylem plates. At their 
first appearance they consist of four cambium strands similar in 
character to those described in the stem, that is, they are com- 
posed of long, small, sharp-pointed cells arranged in groups | 
each of which presents an oval circumference on the cross-sec- 
tion. These four ovals may meet at the centre, in which case 
there is no pith formed. The first appearance of xylem is pro- 
duced by certain of these cells turning into lasting tissue of the 
nature of xylem. This development or change proceeds at first 
toward the centre, and continues until the entire central portion 
is occupied by xylem, a large portion of which consists of ducts. 
The extreme outer portion of the cambium string remains meri- 
stematic and after the xylem at the centre is completed or 
during this time, it continues to develop or cut off new cells 
toward the centre, which are also in time 
turned to xylem tissue. Between the plates 
of xylem the cells do not at first grow into 
cambium cells to give rise to phloem plates, 
but remain parenchymatic. They do, how- 
ever, become meristematic, cutting off new 
cells so that the section which they compose FIG. 53. 
keeps pace with the radial increase in the aa represent the cam- 
xylem plates. The first appearance of ee es ton 
phloem is caused by some of the cells of eee es 
the outer portion of these sections changing the sections between. 
gradually into phloem elements, chiefly sieve pene aee ee 
tubes, but these are often accompanied by ondary growth. dthe 
bast cells. (Fig. 53.) eae ass 
This gives an approximate idea of the manner of origin of 
a radial bundle. In roots where a pith occurs the process is 
