The Ohio Naturalist. 
[Vol. XI, No. 2 
226 
Upon taking up the present investigation, the writer expected 
to find that the division of the laminae in the different genera, 
was brought about by the same process with of course some minor 
variations. It was then very surprising to find the widest differ- 
ences prevailing among the various genera, differences in some 
instances so great as to make the histological processes involved 
appear diametrically opposed. The end result, the branching of 
the plant, in all cases is the same so we have in these forms a most 
striking example of those numerous instances in nature in which 
a common end is attained through totally different means. 
SUMMARY. 
1. The splitting of the lamina of Nereocystis is due to a rela- 
tive increase of periclinal divisions resulting in the intercalation 
of weak new tissue which is mechanically tom apart. The 
wounds heal by normal growth, building the tissue out and around 
the exposed edges of the medulla. 
2. In Postelsia cell division in the meristematic epidermis 
ceases almost entirely at the point where splitting is to occur and 
the lamina becomes so thin by the continued differentiation of 
the tissue already present that it is torn apart by the impact 
of the waves. The wounds heal as in Nereocystis. 
3. In Lessoniopsis an area within the mid-rib is reduced to 
the thickness of normal lamina by relative increase of anticlinal 
divisions. Within this area further reduction and weakening 
occurs by anticlinal divisions in the cortex. The wounds after 
mechanical ripping are at first covered with callus; later healing 
as in Nereocystis. 
4. Macrocystis shows the perforation to originate by local 
gelatinization of the inner and middle cortex and cessation of 
periclinal activity in the epidermis over the gelatinized portion, 
resulting in a deep sinus on either side of the lamina. The adja- 
cent tissues are finally forced in on the gelatinized places until 
the epidermis breaks apart forming the perforation. Healing as 
in Nereocystis. When once formed the fissure advances by ex- 
cessive cortical meristematic activity which first tears apart the 
epidermis and finally the whole lamina, which meanwhile has 
locally become filled with cortex cells. Healing is by transfor- 
mation of the exposed cortex into epidermis. 
5. In Dictyoneuron only the advance of the cleft was studied. 
An internal wedge-like mass of tissue, a cortical meristem, tears 
the overlying layers apart. Periclinal division of the cells lining 
the gap forms a fan-like structure which reduces the lamina in 
thickness and strength until mechanical tearing follows. The 
superficial cells of the new tissue formed on the respective edges 
are changed to epidermal elements and remain so. 
