JV. On tlie conlinuity of the pcotoplasm through tlie walls of vegetable cells. 05 
cells surrounding the vascular bündle even this difficully is removed, for 
there (lie middle Iamellae become quite invisible. Thev too exhibit the same 
structure, l>ut no slriation can be observed, and moreover on account of 
their great swelling the protoplasmic processes projecling into the pits are 
separated by an appreciable distance from the pit-closing-membranes. 
From the eomplele reliance which may be placed upon the staining of 
Picric Hoffmann’s blue, as a result of very numerous lentative experiments 
I am in a position to state that this peculiar colouration of the pit-membrane 
is due to die presencc of protoplasm in its substance. From a careful exami- 
nation of what takes place in other instances, and from a thorough com- 
parison ol this structure with numerous other structures of a like kind, one 
may furlher state that this staining is in realily caused by and is the ex- 
pression of, a number of delicale protoplasmic filaments traversing the pit- 
membrane after the manner of the threads in sieve-tubes; that these fila- 
menls join on to the ends of the protoplasmic processes entering the pits, 
and that the whole closing-membrane is in fact a sieve-plate. 
ln formulating this Statement it is important that vve should clearly 
understand what are the exact points to be taken into consideration, iii 
making a reliable comparison between this and other lissues. There are 
two principal facts to be dealt with, viz the thickness or thinness of the pit- 
closing-membrane and the size of the threads which traverse it. With a 
thick wall and a fairly thick thread vve have the best possible conditions, 
and the whole structure will be easily seen. This actually occurs in many 
endosperms. Again there is the combination of a well delined thread, and 
a tliin membrane. This state of things, which also occurs in certain endo¬ 
sperms is obviously much less favourable. With a thick wall and a thin thread 
the difficullies increase; and lastly the simultancous occurrence of a very 
thin membrane and a very thin thread gives us the very worst combination, 
in which it is next to impossible, if not impossible altogethcr to see anylhing 
of any structure wliatsoever and owiug to the extreme thinness of the elosing 
membrane so little colouration takes place that we cannot even define any 
staining, which would suggest the presence of protoplasmic fdaments in that 
structure. 
In Mimosa we have to deal with a case of some difficulty, for we have 
thin threads, and a somewhat thin closing membrane, but although the 
combination may be unfavourable it is certainly not at its worst, for there 
is a distinct and well defined colouration, and in addition an appearance of 
slriation. In such tissues as the base of the leaf-stalk of Prunus lauro- 
cerasus, the pit-membranes are thieker, and in addition, the middle Iamellae 
ol many of the cells are quite invisible after swelling. Such a tissue treated 
by the Chlor. Zinc. Jod. melhod slained with Picric Hofpmaxn’s blue, and 
mounlcd in Canada-balsam demonstrales that in the clearly stained pit- 
membranes a distinct slriation, and even distinct threads can be seen. From 
Arbeiten a. d. bot. Institut in Würzburg. Bd. EU. 5 
