300 Sykes. — Anatomy and Histology of Macrocystis pyrifera and 
divisions. The cells of this inner cortex have very dense contents and 
large nuclei. Pits are present in their thin transverse walls, but they are 
not very easy to see, being more numerous, smaller and shallower than 
in any of the other walls. They do not appear, either in the inner or outer 
cortex, to be arranged in a ring as are the pits in the transverse walls 
of the cortical cells in older material. A few pits are also present in the 
longitudinal walls. 
, The innermost layers of primary cortex give rise to hyphae which run 
into the medulla. The free ends of these hyphae were never certainly seen, 
and it seems probable that secondary connexions are formed among them, 
but to find definite proof of this it would be necessary to study their 
development in young plants. 
The medulla at this stage is nearly circular in cross section, and is 
composed of elements which are still more elongated than those making 
up the inner cortex, and have also more swollen walls. Between these 
elements, and in the mucilaginous substance formed from the swollen middle 
lamella of their walls, run numerous, much branched hyphae. In a trans- 
verse section at this stage there is no definite boundary line between the 
medulla and inner cortex, and the elements of the medulla themselves 
do not appear to be sharply differentiated as to size or other characters ; 
but in longitudinal section it is seen that those forming the one or two 
outer rows pursue a considerably straighter course than the others, and 
are also somewhat wider, having more dense contents and larger nuclei. 
The elements composing these outer rows are the first of the true sieve- 
tubes, but at this stage are quite immature. 
The central region of the pith or medulla is occupied by numerous 
sieve-tube-like elements, the ‘ trumpet hyphae * of Oliver. These are 
presumably the original central cells of the thallus which have become 
stretched by the growth of the outer tissues and separated by the hyphae 
growing in from the cortex and anastomosing between them, but never 
forming connexions with them. These central elements, which it will 
be useful to call the ‘ primary pith filaments ’, though they do not pursue 
a very straight course, are yet only seldom branched, except indeed 
at the origin of a leaf segment. At such a point it is usual to find numerous 
examples both of pith filaments and sieve-tubes dividing into two, one 
branch continuing its course in the main axis, while the other turns off into 
the leaf. Callus is apparently formed very early in the central part of the 
medulla, and even at this stage it is not unusual to find long strings of 
callus in course of formation. It seems probable, however, that in a slightly 
younger stage these elements must have functioned for conduction, but 
that, after the development of the sieve-tubes from the inner cortex, they 
have acquired a mechanical function, since their lumina have become 
obliterated by callus. 
