Dec, 1910.] Self- Dividing Laminae of Certain Kelps. 221 



tissue changes involved are confined to the region immediately 

 around the advancing fissure. 



The splitting process in Postelsia cannot be correlated with 

 that of Nereocystis. Instead of a mass of new tissue being formed 

 by periclinal activity, there obtains a relative lessening or inhibi- 

 tion of all cell division, while the modification of the various cells 

 in the dividing region into cortex and pith-web continues unabated. 

 Thus the lamina locally becomes thinner and thinner until the 

 critical point is reached. Fig. 9 shows a nomial portion of a lami- 

 na at one side of the splitting region. The epidemial and hypo- 

 dermal cells are markedly elongated perpendicular to the surface ; 

 the larger middle cortex cells have their usual isodiametrical form 

 and the inner cortex and pith-web are stretched horizontal to the 

 surface. Contrasting with this is Fig. 10 froni the middle of the 

 furrow on the same section as Fig. 9. Here the large cortical cells 

 have become prematurely elements of the middle layer and even 

 the outer cortical elements show evidence of horizontal stretching 

 due to the expansion of the superficial layers, while there are few 

 divisions in the epidermis. 



This lack of meristematic activity fails to build out the lamina 

 to the normal thickness as .shown in Fig. 9, and reduction in thick- 

 ness continues progressively as the cortical cells are stretched into 

 the weak and yielding pith-web elements. In Fig. 11 the lamina 

 is shown at the critical point where ripping apart may occur. The 

 cortex has practically disappeared. The development of the 

 remaining hypodermal cells has parted the inactive epidermis on 

 each side and the lamina is now in condition to be torn apart by 

 the slightest twist. Throughout the entire process of reduction 

 to the critical point no cell gelatinization occurred. After fission 

 the wounds are healed exactly as in Nereocystis, by activity of the 

 adjacent epidermal cells building tissue out and around the exposed 

 edge of the medulla. 



LESSONIOPSIS. 



The lamina of Lessoniopsis is characterized by a thickened 

 mid-rib made up chiefly of sclerenchymatized cortex. Upon 

 division the reduction of the lamina to the critical point may be 

 divided into two rather definite stages. In the first place as seen 

 in the development of the perforation, broad furrows are formed 

 in the basal portion of the mid-rib, resulting in the intercalation 

 here of a small area of normal undifferentiated lamina. Then 

 through this, rather than through the mid-rib proper, the cleft 

 is jjropagated. These two stages are more sharply differentiated 

 in the case of older and advancing splits, for here the portions of 

 the divided normal lamina or the reduced mid-rib retain their 

 thin blade-like character and broaden out until the daughter 

 laminae are symmetrical and the mid-ribs occupy their nonnal 

 median position. 



