Laminaria saccharina. 
307 
plate and then proceed towards the centre, the periphery being therefore 
generally more advanced in development than the centre ; they also 
always begin at the ends of each of the threads of a group and work 
towards the middle lamella. Sometimes it was found, as in Figs. 14 and 15, 
PI. XIX, that callus formation had begun on one side only of the plate in 
any given group, and had gone as far as the middle lamella from that side, 
while on the other side unaltered threads were present, as yet not even 
changed with regard to their staining properties. 1 
In many cases each thread can be clearly seen to have its own callus- 
rod, but the separate callus-rods soon fuse, and a single rod is formed, 
through which the deeply staining threads are seen to pass. These threads 
then disorganize and, together with the intervening callus, give rise to a 
single slime-string. Numerous preparations, such as those shown in Figs. 15, 
16, PI. XIX, were obtained in which callus and slime-string production 
had proceeded a certain distance from each side of the sieve-plate, and 
unaltered threads, or threads each enclosed in a separate callus-rod, could 
be seen stretching between the cones thus formed. In some sieve-plates 
(Fig. 16, PL XIX) this stage of development had been reached on the 
periphery, while in the centre groups of unaltered threads are still present. 
The middle lamella then becomes dissolved, at the point at which two 
cones finally meet, and a continuous slime-string enveloped in a tube of 
callus is thus obtained ; the perforation of the sieve-plate is now complete. 
The stages in the obliteration of the innermost sieve-tubes are more 
clearly seen in longitudinal than in transverse sections. In Fig. 1 6, PL XIX, 
callus formation can be seen to have spread, from the heads of the rods 
surrounding the slime-strings, over the interveningareas. In Fig. 17, PL XIX, 
a small pad of callus has been formed over the central portion of the sieve- 
plate, and in Fig. 19, PL XIX, such a stage as this has given rise to a large 
mass, extending right across the sieve-plate, and filling up a large portion of 
the sieve-tube. From this it . seems clear that the obliteration, unlike the 
development of the sieve-plate, proceeds from the centre towards the 
periphery of the plate. In Fig. 18, PL XIX, a thin layer of callus is 
represented in process of being laid down against the wall of the tube ; 
this layer soon increases in amount, generally starting from the sieve- 
plates, but sometimes arising separately in different parts of the tube 2 . 
Finally we obtain a stage such as that represented in Figs. 19, 23, 
PL XIX, in which the lumen is almost entirely obliterated. 
iii. Lateral sieve-plates. On the lateral walls of the sieve- tubes, the 
places at which branching has occurred, or from which lateral connexions 
have originated, develop normal sieve-plates (Fig. 10, Pl. XIX). Sieve- 
plates are also developed on the cross walls of the rows of cells forming 
the cross connexions between the sieve-tubes. In many of these cases the 
1 Cf. Hill, 1 . c., 1901, II, Pl. XXXIII, Figs. 18, 23. 2 Oliver, 1 . c., 1887. 
