aquilinum , with some Notes on Mcirsilia quadrifolia , £/c. 583 
tube is still functional, and the presence and behaviour of the mucilaginous 
droplets, which still remain somewhat of an enigma. The fact that the drop- 
lets or granules occur in Monocotyledons also, and that in the early stages 
of the development of the sieve tubes in Vascular Cryptogams they cannot be 
distinguished in the granular mass of protoplasm forming the contents of the 
sieve tubes, seems to suggest that their manifestation may be connected 
in some way with the prolonged functioning of the sieve tubes. Their 
general behaviour and appearance suggest that they are viscous drops of 
slime akin to the substance of the slime strings, but whether they really 
function in the formation of the slime string, or whether they merely sink 
mechanically into the pits left by the solution of the callus, cannot be 
affirmed absolutely ; their final fate would certainly appear to be incorpora- 
tion in the substance of the slime string. It is possible that they are attached 
to it ab initio , and are at first very small, and that afterwards they gradually 
swell until their relationship to it is easily discernible. In that case they 
would seem to be simply the swollen heads of the slime strings. The 
separate individuality of the mucilaginous droplets would naturally become 
more clear as the parietal layer of protoplasm becomes thinner, consequent 
upon the increase in size of the sieve tube. 
The constant connexion of the droplets with the sieve plate was noticed 
by both Russow 1 and de Bary , 2 and the latter figures the droplets as 
connected with one another by threads crossing the sieve plate. 
With regard to the callus, it cannot be disputed that there is no very 
great development of callus in Vascular Cryptogams. In the life-history of 
each sieve tube, however, in Pteridium , at any rate, there is a stage at which 
callus is quite abundant and easily detected. An observer, examining 
material in the stages before callus formation or after its dissolution, would 
be disposed to say that callus was absent, and this no doubt led to the 
contradictory statements that were for a long time made upon the sub- 
ject. The stage in which callus is present does not seem to last very long ; 
the amount of the rhizome of Pteridium in which this stage is to be found 
would therefore be comparatively small in relation to the total mass, and the 
chance of the region containing callus being overlooked would be consider- 
able. The same is true of Marsilia quadrifolia ; many sections were exam- 
ined, and the callus stages were only present in a few. No stages with 
callus were found in Lygodium dichotomum , but the material examined was 
all derived from petioles of approximately the same age, so that the absence 
of callus was not surprising ; examination of a sufficient quantity of material 
of varying ages would very likely reveal it. Boodle records his inability to 
detect callus in the Schizaeaceae, attributable probably to the same cause, 
as the general appearance of the sieve tubes in Lygodium , though they are 
1 Russow (’ 82 ), p. 209. 
Qq2 
2 De Bary (’84), Fig. 79 B. 
