384 Farmer and Hill \ — Arrangement and Structure 
ever, the protoxylem clearly extends outwards as far as the 
endodermis, and even in the case of those which exhibit the 
exceptional features just alluded to, it commonly happens 
that it does not affect all the xylem rays of the root. 
The. transition from the root to stem is accompanied by 
a rearrangement of the vascular constituents of the stele. 
The diarch xylem-plate loses its definite outline, owing to 
a considerable increase in the number of its tracheids that 
appear on the sides of the plate (Figs. 9, 10), which thus causes 
the wood to become roughly circular in transverse section ; at 
the same time the protoxylem loses its individuality and 
thus the diarch character becomes entirely lost or obscured. 
The phloem also extends over the whole periphery of the 
wood, and in this region it consists chiefly of parenchyma, 
the small cells of which are very clearly defined by reason 
of their striking protoplasmic contents and large nuclei. 
Sieve-tubes may be detected here and there, but they are 
not numerous, and they are very different from the large 
and well characterized sieve-tubes in the later-formed parts 
of the stems of older plants. They appear in these young 
stems as elements with thick and densely staining walls, and 
possess but little contents, and are thus clearly distinguishable 
from the adjacent phloem parenchyma. The sieves on their 
walls are very plain when examined in longitudinal sections. 
Callus, however, so far as we have been able to discover, is 
absent from the sieves. 
When young plantlets are examined, the tissue lying just 
outside the xylem is seen to be in a state of division and 
new elements are in this way produced, which may retain 
their parenchymatous character, and thus more widely sever 
the protophloem from the wood, or some of the cells may 
undergo further change and become converted into tracheids 
which thus reinforce the primary wood of the stele. It is 
easy to find in suitable material all stages in a process which 
represents the commencement of a secondary thickening in 
these plants. In older specimens this phenomenon assumes 
a more striking aspect, as we shall describe later on. 
