Dec., 1910.] Self-Dividing Laminae of Certain Kelps. 
2 2 I 
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 normal portion of a lami- 
na at one side of the splitting region. The epidermal and hvpo- 
dermal cells arc 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 from 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. 
LESSONIOrSIS. 
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 propagated. 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 normal 
median position. 
