202 PROCEEDINGS OF THE AMERICAN ACADEMY 



when viewed from one surface, or a depressed one when viewed from 

 the oilier face of the lamina. 



The microscopic study of the details of the development of the per- 

 forations has required only sufficiently thin sections of their various 

 stages. Free-hand cutting wholly failed to give these. A piece taken 

 from the base of a lamina, and containing several early stages in the 

 development of perforations, was then imbedded in paraffiue and cut 

 into very thin sections by means of the microtome. The sections 

 were then mounted in balsam, according to the usual method for thin 

 sections of animal tissues. Although this seems a hard treatment for 

 an algal tissue, the tough, leathery character of the frond enabled it to 

 withstand well, and very satisfactor}' results were obtained. 



Sections jirepared in this way show that a perforation originates as 

 a depression of the epidermis of either of the surfaces of the lamina. 

 (Fii;-. 6, a.) This depression is formed by a rapid multiplication of 

 the cells of the epidermis in that region and the incurving of the layer 

 in consequence of its increased length. (Fig. 7.) After the inward 

 bending has begun, the continued growth of the epidermis causes it 

 to press deeper and deeper into the underlying tissue, which is com- 

 pressed and pushed aside, until the medulla is reached. (Fig. 6, b.) 

 The depression has, at this stage, the form of a basin with sloping 

 sides and a nearly flat bottom, lined by the unbroken epidermal 

 layer. The l)ottom of this depression lies against the medullary tis- 

 sue, while its sides are in contact with the compressed cortex and form 

 angles with the medulla. 



Further growth appears to take place only in that portion of the 

 epidermis which forms the sides of the depression, and has two effects. 

 Since the bottom of the depression remains passive while the sides 

 are tending to push on into the medulla, the former is stretched with 

 increasing force until a rupture occurs near its middle ; this whole 

 bottom layer of epidermis now dies and falls away, leaving the sides 

 unrestrained. Under the influence of their rapid growth, the latter 

 press into the mrdnlla like a circular punch, cutting out that part of 

 the medulla formerly in contact with the bottom of the epidermal 

 depres.-^ion. (Figs. 6, c, and 8.) Meanwhile, that part of the cortex 

 of the other surface of the lamina which corresponds in position to 

 these lost tissues has begun to show a changed appearance. Although 

 not directly affected by the growth of the depression just described, 

 it becomes dark and shrivelled in appearance (Fig. 8), and soon 

 breaks away from the living cortical tissue surrounding it, escaping into 

 the water as a small block of dead and collapsed cells. (Fig. G, d.) 



