Laminaria digitata and Laminaria saccharina. 187 
During this formation of the medullary tissue, the stipe becomes more 
rounded and increases in length and thickness. Then a disk-shaped expan¬ 
sion develops at the base, it is slightly convex in the direction of the apex 
of the plant, and partially covers over the rhizoids (Fig. 10). The next 
stage consists in the outgrowth from the disk of thick circular processes with 
rounded ends, which extend along the surface to which the young plants 
are attached (Fig. 11). These processes form the hapteres, by means of 
which the plant gains permanent attachment to the rocks. At first they 
exceed the stipe in thickness. They consist of the limiting layer, cortical 
cells, and a well-developed medullary layer. The cells of the latter arise 
from the growing point at the base of the stipe in the same way as the cells 
of the medulla in other parts of the plant (Fig. 15). Thus strands of 
medullary tissue can be traced, which radiate from the growing point 
through the tissues of the disk to each haptere. 
The gradual upgrowth of the medullary tissue in the young plant can 
be seen to correspond to a darker area in the lamina visible to the naked 
eye on holding the plants up against the light. In some cases, the uni¬ 
cellular layer at the apex and periphery of the lamina degenerated and 
became separated, without apparently much affecting the after-growth 
of the plant. 
If the young plants be removed from the attachment which they have 
obtained by means of their primary rhizoids, they do not readily again take 
root. In this detached condition the stipe shows marked negative helio- 
tropism. If the plants be placed in a dish standing on white paper, where 
most of the light comes from below, the stipe soon turns upwards away 
from the light, whilst if the dish be placed on black paper, the stipe turns 
downwards and the rhizoids may again find attachment to the glass. 
In impure cultures, filaments of Ectocarpus were nearly invariably 
found entangled among the rhizoids, but they can be distinguished by the 
fact that the rhizoids of Laminaria possess no chromoplasts. 
It is impossible at present to say how far the development in artificial 
culture solutions resembles that in nature, but it is probable that all processes 
are somewhat accelerated. Assuming that the young plants found growing 
in the Aquarium tanks in July arose from those kept in the tanks in the 
preceding November, the rate of growth in the artificial solution would 
appear to be nearly three times as rapid as that in the tank water. In one 
case a piece of Laminaria saccharina was found on which some spores 
were developing, and the young plants thus formed resembled in every 
respect those grown in cultures. 
All that has been said with regard to reproduction and development 
applies both to Laminaria digitata and Laminaria saccharina with the 
following small exceptions :—In the youngest stages the lamina of L. sac¬ 
charina consists usually of a single row of cells, while that of L. digitata is 
