256 Hill . — The Histology of the Sieve-Tubes of Angiosperms. 
account 1 of the present research with reference to the structure of the young 
sieve-fields may require modification, but all such questions will be referred 
to in the course of this paper. 
Summary : — From the review of the literature presented in the previous 
pages it will be seen that more than one account of the developmental 
history of the sieve-plate and other questions of phloem histology has 
been given during the last thirty years. Wilhelm’s work, as far as it 
goes, agrees with the results of the present research, and had he been in 
command of a better technique there can be little doubt that he would have 
been able to work out the finer histological details. Nearly all his work 
can be accepted as it stands, and serves as the foundation on which the 
complete history may be built up. 
The importance of Russow’s work, taken in conjunction with Wilhelm’s 
results, lies in the fact that the relation of the slime-strings to the callus- 
rods both in adult sieve-plates and in sieve-fields (lateral pits) is definitely 
established. Russow also contributed to the early history of the sieve- 
plate, and his description of the first appearance of the callus is apparently 
in exact agreement with the results of the present research. 
The opposed views of the formation and development of the callus by 
these two botanists are probably both partly true, for, as will be shown 
later, it seems likely that the earliest formation of the callus is due to a local 
transformation of the cell-wall, whilst the activity of the protoplasm is 
responsible for its subsequent increase. 
Lecomte, influenced no doubt by Mangin’s work on the cell- wall, 
produced a totally different theory of the origin of the sieve-plate of a 
complicated character, since the young end-wall is considered as being 
composed of two different substances forming a meshwork and interstitial 
matter respectively. The latter substance is considered in some cases to 
be gradually transformed into slime-strings, whilst in other plants there is 
said to be no open communication. Lecomte’s views on callus formation 
are not very clear, but he agrees rather with Wilhelm in supposing that it 
is due to a change of part of the pit-closing membrane. On the whole 
Lecomte’s work, from the histological point of view, does not advance our 
knowledge of the sieve-tubes to any very valuable extent, nor has he thrown 
any light on the presence of connecting-threads in the tissues of the phloem. 
To Kuhla and Fischer we owe some knowledge about protoplasmic 
connexions between sieve-tubes and other phloem tissues, &c., but, beyond 
the fact that they have demonstrated the existence of such threads, their 
work cannot be considered as very trustworthy, owing to the nature of the 
methods which they employed. 
Strasburger has attempted to carry on the investigation of the 
histology of the sieve-tubes from the point where it was left by Wilhelm 
1 Hill, Ann. Bot,, vol. xvii, 1903, pp. 265-7. 
