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



denced by the increased number of pith-web elements, together 

 with the incipient cell rows from which they were derived. The 

 unusual number of cortex cells formed rai^idly stretches and atten- 

 uates the pith-web and inner cortex until the original medulla, 

 localh', has been almost replaced by the newer and but slightly 

 modified cortical elements. F'ig. 3 illustrates the process in a 

 more striking manner. This as well as the remaining figures are 

 of the same magnification. At this stage periclinal divisions have 

 been so rapid that small ridges have been formed in the middle 

 of each broad furrow. The enlarging inner cells give the cortical 

 structure a fan-like aspect. The inner cortical cells in the middle 

 region pass into the middle layer before they reach their maximum 

 size due to the rapid development of the cells over them. This 

 accounts for the local massed condition of the medulla in the 

 splitting region, causing the dark splitting line when the blade is 

 viewed by transmitted light. The final result of all this activity 

 is the intercalation of a region, made up entirely of new tissue, 

 which has no strength to resist wave action and is easily torn apart. 

 Fig. 4 shows the lamina at the critical point with the tear partly 

 through it. The inner cortex cells of the preceding figure have 

 passed into the middle layer condition and the lamina is markedly 

 reduced in thickness. When severely whipped the laminae are 

 often ripped at their distal ends; the tear if of any depth always 

 following the weakened zone of the splitting line (Fig. 1.) 



The wounds fomied are shown in Fig. .5 which in the serial sec- 

 tions was taken from the same slide as Fig. 4. By normal acti\'ity 

 the epidermis and cortex are built out and around the exposed part 

 •of the medulla (Fig. (i), finally coming in contact (Figs. 7-S). Gen- 

 erally the two epidermal layers do not exactly meet and the edges 

 of the new laminae show a scar in section. Subsequent to the 

 healing, the cells of the inner cortex, overlying the edge of the 

 middle layer, do not develop to normal .size but become scleren- 

 chymatized (Fig. S). Growth above presses them inward, notice- 

 ably bending the hyphae of the middle layer. This condition 

 disappears later when the thick walled cortical elements pass over 

 to the medulla. 



This method of splitting was observed in several specimens but 

 in no instance was any deliquescence or cell disintegration observed 

 as reported by MacMillan in his observations on this plant. 



POSTELSI.V 



Postelsia and the other Lessoneatae dift'er from Nereocystis in 

 the absence of a long splitting line fonned in front of the actual 

 cleft. A close inspection shows, however, the presence of a very 

 short line indicating that the modification of the inner tissues is 

 not begun until the fissure is very near. But for the most part the 



