(14 
Walter Gardine«. 
in every case. I have been able lo explain such appearanoes in die follow- 
ing ways. They inay be produced: First, from (he apparent overlapping of 
unconnected threads which lie in a different plane lo that of Ihe coverslip: 
Secondly, when two extremely attenuated threads lie very dose lo one 
another, they may appear to be continuous, and may give Ihe eye Ihe im- 
pression that they really join, although great care and accurate focussing 
will prove that such is not Ihe case: Thirdly, (hat such an attenuated 
thread coining from a main protoplasmic mass may be inlercepted by a 
middle larnella lying above it which is free from il, bul which appears lo 
be pierced by it. Frequently it will be seen that Ihe attenuated process has 
not yet reached ils own pit, and furlher that the process on the other side 
of the pit may be wide at ils extremity, and that it is between diese two 
that the true pit-membrane intervenes. 
Having thus disposed of two of the appearances of protoplasmic con- 
tinuily which may lie produced when such a tissue as that of the pulvinus 
ol Mtmosn is trealed with strong Sulphuric acid and stained, it only remains 
for me lo deal with the last of those mentioned above w hich is moreover 
undoubtedly real namely, the appearance of a sieve-slrueture in the pit— 
dosing-membrane. ln this instance one observes the following appearance. 
Fixing on a favourable case in which two well defined broad protoplasmic 
processes are opposile one another, one can observe that between diese tw o 
darklj stained ends, and traversing the pit-membrane, there is a lighler 
stained area which appears to bridge over the swollen dosing-membrane 
and lo unile as il were the tw o opposile and deeply colourcd processes. The 
form presented by this stained portion is usually that of a flattencd sphere, 
the diameter of each Uattened end being the same as that of the pit-closing- 
membrane, while in the direction of ils greatest breadth it exceeds this 
diameter in consequence of its spheroidal shape. In favourable instances, 
and with a high power, an appearance of strialion may be detected in this 
stained area, the Striae running in a direction parallel lo the long axis of 
the pils, bul making a curve in their course across die pit-membrane instead 
of traversing it in a straight liue. The appearance of strialion is hoWever 
exceptional, and as a rule, nothing more than die colouration, and the form 
that such colouration assumes can be made out. Between two opposite 
processes with pointed or altenuale ends the same structure naturally 
occurs, and when the processes have conlraeted away from the closing 
membrane, it follows that a distinct interval between the deeply stained 
processes and the lighler stained area will occur though in suecessfully 
prepared sections one cannot help being greatly struck with the wonderful 
tenacitv with which die protoplasm clings to the closing membranes of die 
pits. In the parenchyma cells where the middle larnella is well developed 
and resistent, this structure, evon after the prolonged action of glycerine, 
oflfers some slight impediment lo obscrvation, bul in the prosencbymatous 
