226 The Ohio Naturalist. [Vol. XI, No. 2, 



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 torn 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 epidemiis 

 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 b\' 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 epidcmial elements and remain so. 



