Hill. — The Histology of the Sieve- Tubes of Angio sperms. 285 
verted into slime-strings associated with callus-rods, but those of the 
companion-cell wall are continued as ordinary protoplasmic threads. 
18. Similar thread-groups are found in the walls between sieve-tubes 
and bast-parenchyma cells ; they are covered by definite callus-pads during 
the winter, and median nodes appear to be present. 
19. The young bast fibres in the phloem of Vitis vinifera are in 
connexion by means of groups of fine protoplasmic threads. 
20. The details of sieve - tube histology appear to be in general 
harmony in the material examined. The formation of slime-strings from 
protoplasmic threads and their association with callus-rods, as well as the 
origin of the callus both by transformation of the cellulose and by proto- 
plasmic deposition, are found to proceed along similar lines in representatives 
of both Angiosperms and Gymnosperms. 
21. The secondary changes which take place in the connexions 
between the sieve-tubes themselves and between sieve-tubes and other 
elements of the phloem may be explained as follows. In the sieve-tubes 
there is the need of the formation of definite holes for the translocation of 
material, and the connecting-threads serve as useful paths along which to 
work in order to produce tubes of sufficient diameter to enable an actual 
flow of slime to take place. Then, since the large holes produced are 
likely to be a source of danger to the plant when translocation diminishes, 
the callus is utilized to regulate the bore of the tubes, and finally appears 
to put a stop to translocatory processes altogether. 
22. I11 the case of the threads between the sieve-tubes and other 
phloem elements, it is no doubt essential that the means of communication 
should be enlarged to enable the normal physiological processes to take 
place. And it seems likely that the numerous enlarged communicating 
channels allow of the passage of secretions into the sieve-tubes from the 
companion cells. 
23. With regard to the physiological function of the threads it appears 
that they serve primarily for the transmission of stimuli, but that, owing to 
subsequent modifications in the special cases of the sieve-tubes, they become 
enlarged and are able to serve secondarily for purposes of translocation. 
Literature cited. 
Fischer, A. : Ueber den Inhalt der Siebrohren. Ber. d. deutsch. bot. Ges., 1885. 
Ber. Math.-Phys. Kl. d. K. Sachs. Ges. d. Wissensch., 1886, p. 291. 
Gardiner, W. : Proc. Camb. Phil. Soc., vol. v, 1885, p. 230. 
I Roy. Soc. Proc., lxii, 1897, p. no. 
— Proc. Camb. Phil. Soc., vol. ix, Pt. viii, p. 504. 
The Genesis and Development of the Wall and Connecting-threads in the Plant- 
cell. Roy. Soc. Proc., lxii, 1900, p. 186. 
