Laminaria saccharina. 
301 
Stage ii. Commencement of rapid groivth in thickness. In a stage 
slightly older than the one described above, the medulla has become oval 
in outline, and sieve- tubes are being rapidly produced by the stretching of 
the inner cortical cells, while the outer cortex is considerably enlarged by 
the divisions of the external meristem. 
The inner cortex is still sharply marked off from the outer, while 
between the inner cortex and the medulla there is no clear line of dis- 
tinction. 
Every stage of transition between sieve-tube and cortical cell is 
present. The sieve-tubes on the periphery of the medulla consist of short 
elements hardly swollen at the cross walls, while the innermost ones are 
composed of elongated cells much dilated at the cross walls. Branching 
of the young sieve-tubes is fairly common, and cross connexions, developed 
from the pit canals by the stretching of the pits due to the swelling of the 
longitudinal cell-walls, are very easily traced at this level in their various 
stages of development. There is a considerable amount of callus in the 
primary pith filaments, but even in the innermost sieve-tubes callus 
formation has hardly commenced. 
Stage Hi. Sections taken about one and a half inches below the apex 
show a much more advanced stage of development. (Fig. 44, PI. XXI.) 
The cortex is now about four times the breadth of the medulla, and is 
no longer sharply differentiated into regions, but there appears to be 
a gradual transition, from the outer parenchymatous to the more elongated 
inner elements; the latter give rise to numerous hyphae. The whole 
of the characteristic inner or primary cortex described above 1 has been 
transformed into sieve-tubes, and all the cortex now present is of secondary 
origin, derived from the repeated divisions of the outer cortical meristem. 
Young sieve-tubes are still present on the outer edge of the medulla, 
but it seems improbable that new ones are being formed at this stage. The 
zone of sieve-tubes now contains from seven to nine elements in each radial 
row, and the development of the sieve-plates is rapidly proceeding, callus 
being already present in the innermost rows. The * primary pith filaments ’ 
all contain callus. 
Stage iv. In a stem of about twice the thickness of that last described 
but little change appears to have taken place. 
The almost isodiametric cells of the outer cortex have now deeply 
pitted walls, and the pits are distributed fairly evenly over the longitudinal, 
and arranged in a ring in the transverse, walls. In the more elongated and 
thicker-walled cells of the inner secondary cortex the pits are also arranged 
in a ring in the transverse walls, but in the longitudinal walls they are very 
few and isolated. The appearance of these latter cells, as well as the 
1 pp. 297-298. 
