84 PROCEEDINGS OP THE AMERICAN ACADEMY 



described, in considerable detail, the structure and development of 

 certain of the Dictyotaceaj found in the Bay of Naples. In general, 

 it may be said that the Dictyotaceae are thin, flat, membranaceous, 

 olive-brown algfe, usually very simple in structure, having tetraspores, 

 antheridia, and certain large ovoid bodies termed by Reinke, Hauck, 

 and others oogonia. In the fronds of the various genera of this group 

 two forms of growth may be distinguished ; one by means of a single 

 apical cell, of which Dictyota is a type, and another by means of a 

 row of marginal cells of which Padina affords a good example. While 

 at first it would seem that these two methods of growth are widely 

 different, closer examination shows that the growing marginal cells 

 practically represent a row of apical cells. 



The apical growth of Dictyota dichotoma, from tbe formation of the 

 first division to the ultimate condition found, was clearly described 

 by Naegeli, and further details were given by Cohn and by Reinke. 

 From the lenticular apical cell a saucer-shaped cell is cut off by the 

 formation of a transverse wall. This latter cell divides into several by 

 the formation of vertical walls, and later each one of the new cells is 

 cut into three parts by walls parallel to the surface of the frond. Thus 

 the three-layered condition of the adult frond is attained, and the 

 subsequent changes consist chiefly of the division of the outer cells into 

 many parts to form a cortex. 



Padina, growing as it does in most European waters, has been 

 abundantly studied, and was for this reason cited as an example of the 

 marginal form of growth, but it differs from other Dictyotacese in that 

 the edge of the frond is inrolled. The growing cells which form the 

 margin of the frond of Padina Pavonia, Lam'x, may roughly be 

 said to be brick-shaped, with their anterior faces rounded, and their 

 longitudinal axes placed at right angles to the margin of the frond. 

 From them, as in the case of the apical cells in Dictyota dichotoma.^ 

 cells are cut off by the formation of transverse walls. These newly 

 formed cells almost immediately divide horizontally into two parts, 

 one of which grows in area, the other in thickness, so that the edge 

 of the frond gradually curls over on itself, until the inrolled condi- 

 tion is attained. For a considerable space the frond is but two layers 

 of cells in thickness, for the third layer, which is formed by the divis- 

 ion of the thicker layer parallel to the surface of the frond, does not 

 make its appearance at once. The increase in breadth takes place 

 from the longitudinal division of the marginal cells, an occurrence 

 that is very common in a young, rapidly growing frond. Besides the 

 large, thin, fan-shaped form of the thallus of Padina usually found. 



