394 PHYSIOLOGY 
Phloem strands. The phloem strands are usually definitely related 
to the xylem strands (which carry water), though they occur also inde- 
pendent of them. In most seed plants there is a phloem strand lying 
along the outer face of a xylem strand, and except in the monocotyledons 
there is generally between them a meristem (cambium), which may add 
to the radial diameter of both xylem and phloem. It may also, if it 
extend from one strand to another around the axis, produce new second- 
ary phloem strands between the old ones. The phloem strands form 
a continuous system, and may be traced from the stem outward into the 
leaves and downward into the roots. So followed, they usually dis- 
appear before the xylem strands end; that is, their differentiation does 
not begin so early in the rootlets nor extend so far in the leaves. 
Elements of phloem. The elements of the phloem strands are sieve 
tubes, companion cells, cambiform cells, and parenchyma, with some- 
times mechanical tissues, though the latter belong more commonly to 
the adjacent tissue systems. It is impossible to specify the precise 
role of each of the elements; but among them all the sieve tubes may 
be considered the chief lines of conduction, the others being supplemen- 
tary thereto. 1 In a way the sieve tubes are analogous to the tracheae of 
the xylem; particularly in that, having their end walls partially resorbed, 
they constitute tubes through which the foods may move without the 
delay necessitated by osmotic transfer from cell to cell. 
Evidence of conductivity. The reasons for assigning conductive 
functions to the phloem strands are chiefly these: (i) The pith is so 
commonly dead and its cells filled with gases that it may be excluded from 
consideration. (2) The cortex, too, is often dead; particularly is this 
almost universally trtie of the older parts of shrubs and trees in which 
it is frequently sloughed off after a few years; yet there is an active trans- 
fer of foods. Moreover, the movement of food through the protoplasmic 
membranes of live cells is apparently too slow to meet the needs of plant 
growth. (3) When the cortex is removed by surgical operation, the 
supply of food seems to be quite adequate to permit development; but 
if the phloem strands are interrupted, transfer of foods is almost or quite 
stopped. 
This is particularly noticeable when girdling occurs in nature, as when birds 
destroy a zone of bark in conifers whose wood remains able to conduct water. 
The tops and roots (if one or more circles of branches below the injury remain, 
1 It is as though the sieve tubes were the main railway lines and the adjacent tissue 
sidetracks temporarily occupied. 
