Translocation in Plant Tissues . 
301 
plates, where investigated, have numerous fine perforations each lined with 
a protoplasmic tubule through which the fluid contents of adjacent segments 
are continuous. 
Practically the whole area of the plate may be perforated in this way, 
so that a number of the minute channels occur quite close up against the 
lateral walls of the tube. 
Above the level of the plate the facilities for wave propagation are 
proportional to the cross-sectional area of that part of the protoplasmic 
lining containing the sugar-adsorbing constituents. (Cf. Fig. 2 .) 
By a little consideration of Fig. 2 it readily appears that, owing to the 
existence of a number of perforations close to the periphery of the plate, the 
cross-sectional area of the protoplasmic sheath, which is continuous though 
Fig. 2. Very diagrammatic representation of a portion of a sieve-plate and the walls of a sieve- 
tube to show continuity of protoplasm. Cailose is omitted. Cell-wall, shaded. Protoplasm, white 
(surface), black (in section). 
distorted, suffers little, if any, reduction at the level of the plate. At the 
most it is only as if the tube were provided with a narrow, inwardly- 
projecting ledge, comparable, save for its irregularity, with an annular 
thickening of a vessel. 1 
As far then as its effect upon the propagation of a wave of restoration 
of concentration equilibrium is concerned, the existence of the more central 
portion of the plate may be almost neglected. It allows the free passage 
of various albuminous substances, &c., so that the sieve-plate may be 
regarded as a very satisfactory structural compromise permitting of the two 
processes referred to, and at the same time retaining sufficient mechanical 
strength to guard against the collapse of the sieve-tube through pressure 
exerted by surrounding cells. 
Fluctuations in concentration constantly occur throughout the living 
tissues, especially during periods of vigorous growth and storage. 
1 The sieve-plate should be contrasted with a transverse wall having a single central perforation. 
