584 Hume . — The Histology of the Sieve Tubes of Pteridium 
smaller, does not suggest that their development is likely to differ funda- 
mentally from that in Pteridium. 
The short duration of the stage in which callus can be detected, in 
the sieve tubes of those Vascular Cryptogams examined, confirms the view 
advanced by other authors, 1 that callus can be formed as an early 
stage in the solution of the cellulose of the wall. The existence of callus 
in various states of aggregation or hydration is mentioned by Mangin 2 
and M. G. Thoday ; 3 these different states perhaps represent different 
stages of solution. Deposition of callus directly from the protoplasm 
does not seem to occur in Pteridium , unless the exceptional cases figured 
in PI. LIV (Figs. 10 and 11) originated in that way. 
Summary. 
1. The distribution of the sieve tubes and sieve plates in Pteridium 
aquilinum provides for conduction all round the vascular strand, both in the 
node and internode. In Marsilia quadrifolia , conduction around the stem 
is especially provided for in the nodes, but in the internodes there is very 
little provision for it. The variations in the distribution of the sieve plates 
in the two types are connected with the association together of the leaf } 
roots, and sporocarp at the node in Marsilia , while in Pteridium the roots 
have no regular distribution in relation to node and internode. 
2. The sieve plates in Pteridium aquilinum are traversed at the 
earliest stage by delicate protoplasmic threads, each with a median proto- 
plasmic node. 
3. Callus is developed as a pair of basins in connexion with each 
individual thread, and the middle lamella between the callus basins becomes 
altered to form the median nodule. 
4. The protoplasmic thread begins at the same time to be bored out to 
form a slime string, and at this stage the refringent granules or mucilaginous 
droplets can be seen to be arranged regularly, one at each free end of each 
slime string. 
5. The callus next becomes dissolved away, and the mucilaginous drop- 
lets sink down into the cavities thus left. 
6. Finally, the median nodule becomes dissolved away, and a homo- 
geneous thick slime string is produced, derived partially, as far as can 
be seen, from the paired mucilaginous droplets. 
7. The sieve tubes function for years, but no callus seems to be 
deposited in winter to block them. This is very probably because the 
rhizome of Pteridium is a subterranean organ. Winter material of the 
roots and stems of Vitis vinifera showed callus blocking the sieve tubes 
in the stem, but not in the root. 
1 Oliver (’87), p. 112 ; Hill (’01), pp. 597-600; Hill (’08), p. 278; Thoday (’ll), p. 673. 
2 Mangin (’92), p. 263. 3 Thoday (’ll), p. 3*5* 
