588 Hill . — 77 ^ Histology of the 
enclosing the median nodes which belong to the slime-strings 
(Fig. ix, PL XXXI ; Fig. 20, PI. XXXIII). 
The Development of the Sieve-Plate. 
The development of the sieve-plate can now be considered. 
It is found to be a matter of great difficulty to observe 
( connecting threads ’ in the walls of meristematic tissues, and 
the radial walls of the cambium-cells unfortunately do not 
form an exception to this fact 1 ; and it is not until one 
examines the pit-closing membranes of developing sieve-tubes, 
about three cells away from the well-marked boundary-cells of 
the active phloem, that - threads’ can be observed with certainty. 
The pit-closing membranes in this region are very thin, and 
the ( connecting threads/ which are arranged in small groups, 
are very fine, so that unless great care is taken in the staining 
of the sections, it is almost impossible to see them as fine lines 
at all. Each of these threads bears a small median dot like 
those previously described on the threads of cortical cells 2 3 , 
and small granules are also frequently seen at each end of the 
threads, covering the surfaces of the young pit-closing mem- 
brane and so hiding the true ends of the threads (Fig. 8 , 
PL XXXII). 
As the pit-closing membranes are very delicate, it frequently 
happens that a slight swelling causes a separation of the wall 
to take place at the middle lamella, in which case the small 
median node is seen either on one side of the breach or else 
halved on both sides of the split middle lamella. Usually not 
more than two sieve-plates are seen in this youngest stage, for 
in the next sieve-tube, nearer the phloem, the pit- closing 
membrane is found to be rather broader and to have assumed 
a biconvex or lenticular shape, and to show a secondary pitting 
over its surface so that in sections its edge or margin appears 
slightly crenulated. 
The c connecting threads/ which are arranged in groups as 
1 Threads have, however, been clearly seen in the tangential walls of the 
medullary-ray cells right through the cambium. 
3 Hill, loc. cit., p. 108, Fig. 19, PI. XXXIII. 
