302 
Maugham. — On the Mechanism of 
Such disturbances of the equilibrium at any one region will start waves 
of readjustment, which will be communicated to the sieve-tubes in very 
much the same way as the turning on of a tap in a dwelling-house causes 
a flow of water in the main outside. 
Owing to their special structure the waves could be propagated much 
more effectively in the sieve-tubes, which are continuous from leaf to root, 
than in any other type of plant cell, with the exception of laticiferous 
elements, to which similar considerations might be applied. 
The sieve-tubes are richly provided with connecting-threads on their 
lateral walls adjoining the companion-cells, and the walls of the latter are 
freely connected with the phloem parenchyma cells and with the medullary 
rays . 1 
It is clear then that the views here expressed harmonize well with the 
histological features of the cells concerned. 
The value of connecting-threads for purposes of translocation has 
hitherto been difficult to estimate. Their role becomes very much more 
comprehensible when the above considerations are borne in mind, and it can 
be seen that the statements in favour of their use in translocation, which 
were made by Hill 2 as the result of a study of their distribution and 
frequency, were more justifiable than their author was led to believe at 
a subsequent date . 3 
It will be observed that on the hypothesis here formulated the sugar 
originally coming into existence inside the leaf cells is treated as travelling 
throughout the plant without ever passing through the plasmatic membrane , 
or outer surface of protoplasm in contact zvith the cell-zvall. 
That this layer of the protoplasm in most plants is only with difficulty 
penetrated by sugars from outside (thus allowing sugar solutions to be used 
to effect plasmolysis) has been repeatedly shown . 4 
It is interesting to note that Ruhland 6 found the protoplasm of sieve- 
tubes to be no more permeable to sugars than the protoplasm of other cells. 
Another well-known property of protoplasm is its power to prevent the 
escape of sugar from inside most cells, as in the case of root-hairs, Algae, &c., 
when placed in water . 6 
It is rather a point in favour of this hypothesis of translocatory 
mechanism that it does not require alternating passages of the sugar in and 
out of the protoplasm of adjacent cells through the plasmatic membrane 
and across the intervening cell-wall. 
In this connexion fibres and the ‘stone-cells’ of fruits are instructive. 
The walls of these elements often attain considerable thickness, and require 
1 Hill (1908). 2 Ibid. (1901). 3 Ibid. (1908). 
4 Yeast-cells, saprophytic mycelia, &c., are permeable to sugars. 5 Ruhland (1911). 
6 Cf. also sugar retention by blood. It has been suggested that the sugar is held in combination 
with the proteins of the blood except under pathological conditions, e. g. in diabetes. Cf. Beddard 
(1905), pp. 708-11. 
