300 Maugham . — On the Mechanism of 
to be horizontally disposed and connected ultimately with a main supply 
pipe capable of delivering considerable quantities of water fairly rapidly. 
Thus, if a tap is opened at one of the tanks water will flow out, and 
this will cause a flow from the adjacent tanks and from the others in turn, 
and finally water will pass in from the main. When outflow taps are 
opened at several tanks at once the level of the water in the individual 
tanks at any moment will depend upon the relations obtaining between 
rates of loss and supply for each one and the rate of delivery from the main. 
In the plant the parenchymatous cells with their vacuoles correspond to 
the small tanks, the sieve-tubes represent the main supply pipe, while the 
connecting-threads take the place of the small-bored pipes. 
It is desirable to consider the structure of the sieve-tubes in relation to 
the function here assigned to them. 
In the finest veins of the leaves the phloem becomes reduced to a single 
sieve-tube with companion-cells in connexion with its segments , 1 and finally 
the sieve-tube mother-cell, by failing to divide, gives rise to ‘ transition-cells 
which are very much like enlarged companion-cells . 2 
The transition-cells are in longitudinal connexion with the sieve-tubes, 
while laterally they are contiguous with the cells of the bundle-sheath, as 
also are the companion-cells. 
Soluble assimilates from the photosynthetic system must pass into the 
bundle-sheath. It is highly probable that they then pass into the transition- 
cells, which, being richly provided with protoplasm, would seem eminently 
fitted to exert considerable adsorptive suction, so to speak . 3 
In an ordinary leaf vast numbers of bundle-endings occur, so that each 
need only receive assimilates from a comparatively small number of 
photosynthetic cells. 
Since the transition-cells are in direct continuity with the sieve-tubes, 
the assimilates could next pass into the latter. 
So far, the distance traversed has been very small, equal in fact to the 
united lengths of a few mesophyll cells. The path over which movement 
has occurred in the manner described has included only a very few sets of 
protoplasmic connecting-threads. 
Once the assimilates have entered the protoplasm of the sieve-tubes, 
however, they have a long continuous course practically free from obstacles, 
as will be seen from the following considerations of structure. 
A sieve-tube usually has a very thin 4 wall, and is lined with protoplasm. 
As shown more especially by the researches of Hill , 5 the mature sieve- 
1 Strasburger (1891). A number of figures are given. 
2 Haberlandt (1914), p. 367, Figs. 147 A, 147 B. (Reproduced in Mangham (1910-11), 
Figs. 9, 10.) 
3 Vide infra, p. 305. Cf. Fig. r. 
4 Comparatively thick walls are to be found in many cases. Cf. Compton (1909). 
6 Hill (1901, 1908). 
