Hill . — The Histology of the Sieve-Tubes of A ngio sperms. 257 
and Russow, and has thrown light on the early history of the sieve-plate, 
but owing to his confusion of slime-strings and callus-rods, which is not 
thoroughly cleared up by his review of 1902, 1 his results are shorn of some 
of their value. 
That he is in error about the two different methods of callus formation, 
whether found on the end or on the lateral walls of the sieve-tubes, will, it is 
hoped, be proved in the course of this paper. He also has had nothing 
to say about the possibility of the existence of connecting-threads between 
sieve-tubes and other tissues of the phloem. 
Such then is the state of our knowledge of the histological details of 
the sieve-tubes and phloem of Angiosperms at the present time. It is to 
the general elucidation of the development and character of connecting- 
threads and subsidiary structures in these tissues that the present research — 
commenced in the spring of 1901 — has been directed. A short preliminary 
note has already appeared, but owing to a journey abroad the completion 
of the work has been considerably delayed. Even now there are many 
points which must be subjected to future research, but it has seemed advisable 
to publish the results which have been obtained and to give a general 
account of phloem histology without waiting for the solution of certain 
small outstanding difficulties. 
PART II. 
The Present Research. 
A short preliminary note on the present research was published in 
the Annals of Botany 2 in 1903, and some account of the results was 
given at the Southport meeting of the British Association 3 in the same 
year. The research has been carried out mainly on material of Vitis 
vinifera and Wistaria chine n sis, whilst Cucurbita pepo , Tilia vulgaris , 
Viscum album , and Phaseolus vulgaris and multiform have also been 
studied in greater or less detail. 
As in the case of Pinus , it was found that the nature of the material 
employed was a matter of extreme importance for the study of the develop- 
ment of the sieve-tubes, and it is advisable that the pieces of tissue for 
preservation shall be taken only from the main stems of strong and vigorous 
plants. The young shoots of woody plants in their first or second year of 
growth are seldom favourable. In order to obtain the best results it is 
necessary to cut a piece of the phloem out of a fairly old stem, and care must 
be exercised in order to insure that the cambium is removed at the same 
time. In Vitis , Wistaria , and Tilia the pieces of tissue so removed will 
1 Strasburger, Bot. Zeit., vol. lx, 1902, p. 50. 2 ^Jill, Ann. Bot., vol. xvii, 1903, p. 265. 
3 Brit. Assn. Report, Southport, 1903, p. 854. 
