Translocation in Plant Tissues . 
303 
comparatively large amounts of sugar for their formation. These walls are 
provided with numerous deep pits, through the closing membrane of which 
the protoplasm is continuous by means of connecting-threads while the cell 
is alive. The cells of the date ‘ stone which furnish a reserve of carbo- 
hydrate for the use of the seedling, are especially favourable for observing 
similar features. 
It is surely much more reasonable to suppose that the sugar needed for 
the construction of these walls passes in by way of the connecting-threads in 
the manner above described, than to assume that it passes from cell to cell 
repeatedly through wall substance of ever-increasing thickness. 
In such extreme cases the former supposition seems fairly obvious, but 
it is here contended that sugar exchange is effected in the same way in cells 
with walls of ordinary thickness. 
It should be borne in mind that the length of connecting-threads is in 
all cases determined by the thickness of the pit-closing membrane, and that 
the significance of the thin walls of parenchymatous cells is probably to be 
found in their extensibility, &c., in connexion with osmotic phenomena and 
the maintenance of turgor, rather than in any special facilities they may 
provide for permeation by metabolites . 1 
As mentioned above Czapek stated that plasmolysis of a portion of 
a petiole did not prevent the removal of assimilates from the leaf. 
Deleano 2 considered that the procedure adopted by Czapek gave no 
guarantee that the more central parts of the petiole became plasmolysed, 
and he concluded that translocation through plasmolysed cells was not 
possible. In his own experiments, however, he does not appear to have 
made certain that the cells were not killed by the plasmolysing solution. 
It may be remarked that even when fairly strongly plasmolysed the 
linings of consecutive sieve-tube segments would be much less liable to 
become severed than would those of any of the other cells present. In 
spirit material, for example, the protoplasmic lining is usually contracted 
about the middle region of each of the segments, but is continuous through 
the sieve-plates. 
If translocation is effected in the manner here suggested, it is hardly 
likely that plasmolysis (short of killing) would interfere with the efficiency 
of the sieve-tubes, since the continuity of their protoplasm would probably 
remain unbroken between the normal portions of petiole above and below 
the part plasmolysed. 
An interesting histological feature which lends support to these views is 
1 In specialized absorptive organs, however, thin walls obviously facilitate the entry of sub- 
stances from outside, e. g. from the soil solution in the case of root-hairs. Gaseous exchange, too, is 
similarly aided. 
2 Deleano, 1 . c. Czapek, however, apparently satisfied himself on this point ( 1 . c., p. 146), and 
also made sure that the tissues recovered their turgor when the plasmolysing solution was replaced 
by water. 
