124 ELEMENTS OF PLANT ANATOMY. 
similar, but the four cambium clusters do not meet at the centre, 
thus leaving a cylinder of ground cells which form the pith. 
This also illustrates the so-called primary growth in all cases 
whether of annuals or perennials. At this stage the meri- 
stematic cells le at the outer part of the xylem plates and 
within the phloem elements. Now, when secondary growth is 
about to take place it is first indicated by the formation of four 
new xylem plates which are developed from the meristem inside 
the phloem. These plates like the old, are distinguished 
by large ducts. A continuous cambium ring is now formed by 
the meristem of each phloem-plate gradually extending itself 
till it meets that of the xylem plates. This ring develops new 
cells both centripetally and centrifugally, those toward the centre 
forming xylem elements, those toward the circumference, 
phloem. From this time on the process is like the secondary 
growth of the stem, the various elements exactly corresponding, 
except that in the root there are no primary medullary rays. 
When this growth is continued from year to year, the radial in- 
crease in the central portion of the root demands a corresponding 
one in the tissues of the circumference or rind. This is brought 
about by the formation of a periderm followed by bark. As this 
erowth differs in no essential manner from that of the stem, 
known as secondary growth in thickness, farther description of 
it will be given in connection with this subject. The roots of 
the gymnosperms are similar in all essential particulars to those 
of the dicotyledons. | 
In very many plants the primary root dies early, and its 
place is supplied by side roots. These rise endogenously, that 
is, they arise from a little cluster of cells inside the endodermis 
of the central cylinder. These become meristematic, grow and 
push their way through between the other tissues and finally 
break through the epidermal layer. The bundles of the side 
root are connected with those of the mother axis in various 
ways ; hence arise a number of different types in this respect. 
