Development of Laminaria bulbosa i L amour. 53 
with a thicker outside wall resembling a cuticle. The cells 
are densely filled with chromatophores, and usually appear 
to contain one nucleus; not unfrequently they are seen to 
be divided by thin tangential and radial walls. Further, 
the cells immediately beneath the epidermis are often 
evidently derived from this layer by a similar division of 
cells. But the cells of the third layer from the exterior 
have usually so grown in size and altered their shape that 
it is difficult in many cases to trace their origin. This 
fact seems to indicate that the growth of this Laminaria 
by the formation of new cells is exceedingly slow. Whether 
this be so or not there are no indications of the presence 
of any other meristematic zone in any specimen I have 
examined, such as that readily observable in a transverse 
section through the stalk of Alaria. 
In passing toward the centre of the stem the cells increase 
in size, retaining their general rectangular form. Such cells 
form the cortex, and are more or less sharply marked off 
from the central strand of tissue, which has been termed 
the medulla. In the latter layer the cells become elon- 
gated, usually in the direction of the long axis of the part ; 
and they frequently assume an undulating course. Fig. 12 
represents a portion of the medulla taken from the stalk 
of a young specimen just above the ridge. It will be seen 
that the undulating course is accompanied by a great increase 
in the thickness of the longitudinal walls of the cells. The 
section would show a number of tubes running side by side 
through a clear matrix. The pits connecting neighbouring 
tubes assume the form of short tunnels, and very early the 
cells send out processes into the thickened walls. These 
become hypha-like outgrowths, and penetrate the substance 
of the walls in all directions ; they become divided by 
transverse walls and form a felted network, which, in trans- 
verse section, may assume the appearance of parenchymatous 
tissue. The formation of thi£ hyphal tissue is another item 
in the growth in thickness of the plant, and is also met 
with in many other seaweeds. It is interesting to note 
