156 Scolt. — On some recent progress in our 
On the sieve-tubes and soft bast generally an immense 
amount of work has been done ; of purely anatomical results 
Wilhelm’s discovery of horizontal sieve-tubes traversing the 
medullary rays and Fischer’s observations of a transitory 
system of sieve-tubes external to the bundles in Cucurbi- 
taceae are of special interest The latter author has also 
taught us how to study the contents of the sieve-tubes in 
their natural state. 
It seems that the characteristic contents of the sieve-tubes 
appear when their members are still closed cells, a point in 
favour of Sachs’ view, that they may be seats of metabolism 
rather than mere channels of conduction. May I suggest as 
an hypothesis, that the companion-cells may be secretory , 
supplying the sieve-tubes with those ferments by which the 
starch-grains, and perhaps proteids also, are brought into a 
soluble condition? In the first place, the companion-cells, 
with their exceptionally dense granular protoplasm and con- 
spicuous nuclei, have all the characteristics of secretory cells, 
such as those of nectaries, water-glands, and the epithelium 
of resin-canals. Secondly, we know from Fischer’s work that 
the companion-cells communicate with the sieve-tubes by 
protoplasmic strands. Thirdly, we know that ferments are 
actually present in the sieve-tubes, as is shown by Russow’s 
observation of the peculiar reaction of their starch grains 
indicating diastatic action. In those plants where the 
companion-cells are numerous, the comparison with an epithe- 
lium at once suggests itself. 
Within the last three years attention has specially been 
directed to sliding growth or the changes in the form and 
relative positions of cells during their development. This 
process, which plays so important a part in the formation of 
most tissues of all highly organized plants, had previously been 
almost ignored. The subject has been dealt with already 
elsewhere ; I will only say a word as to a special case bearing 
on this question. The secondary xylem of Monocotyledons, 
such as Dracaena , consists chiefly of very elongated elements, 
usually classed as tracheides. Krabbe has maintained that 
