2 24 The Ohio Naturalist [Vol. XI, No. 2, 



a few hypodermal and outer cortical cells begin to divide rapidly 

 forming a wedge-like mass, which tears the epidennis apart and the 

 process which succeeds in dividing the lamina is begrm (Fig. Ki.) 

 The cells exposed by the parting of the epidennis become passive 

 and subject to the tearing tendencies of the rapidly expanding 

 tissue beneath them. They are separated and in this manner the 

 cleft is carried clear through the lamina. So great is the meris- 

 tematic activity that before the cleft reaches the pith-web this 

 layer locally has been entirel}" replaced by dividing cortical ele- 

 ments, through which the cleft is propagated. The final separa- 

 tion of the last thin walled cortical cells is of course mechanical. 

 By the continual extension of this cortical activity distally, the 

 whole lamina is finally divided, while proximally, the separation 

 is carried some distance down the stipe by the same sort of activity 

 except the meristematic wound tissue is formed in larger masses 

 and the cleft advances in a more irregular manner. 



In healing, the superficial cells of the exposed wound tissue 

 arc transformed into epidermal elements. There is however a 

 tendency to close the wound as previously described, by the 

 crowding or jDressing around of the tissue adjacent to it. 



Material containing clefts of proper age to show the transition 



stages, by which the initial gelatinization process gives way to the 



secondary process of cortical activity, was not available so this 



ntercsting phase of the i)roblem cannot be taken u]) in the ]3rcsent 



diiscussion. 



DICTYONEURON. 



In Dictyoneuron only the method of advance of the older cleft 

 was studied, as the collection contained no material showing the 

 incipient or perforation stages. The process involved in the 

 advance of the cleft was essentialh' the same as that in Macro- 

 c^'stis but the cortical meristem is more definitely localized than 

 in that genus and only occurs at first on one side of the medulla. Fig. 

 17 shows a section of a young lamina in which a split o mm. in 

 length was present. The half of the section not shown was nor- 

 mal like the region at the edges of the drawing. Cell division 

 and growth in the cortex has resulted in the fonnation of a mass 

 of tissue which presses slightly into the i)ith-web. When this mass 

 has become somewhat more extensive than that figured, a few cells 

 near its center begin dividing very rapidly and build up a new 

 secondary mass within the first (Fig. IS), which pushes out the 

 older cells on all sides of it, notably below into the pith-web. 

 On account of this rapid internal division, the original epidermis 

 is pulled apart from a to b and the beginning of the cleft has been 

 started by the wedging action of the ball like mass of new tissue. 

 This cleft shown at Fig. IS, c, next enters the central mass and 

 passes ra])idly to its center. After the develo]3ment of the cleft. 



