OF ARTS AND SCIENCES. 87 



by Professor Farlow in Bermuda. From dried specimens the frond 

 is seen to be more or less fan-shaped, though not so markedly as in 

 Padina, for it is attached by a broader base. When youug the frond 

 is prostrate, being fastened to the substratum by the rliizoid-like hairs 

 on its under surface. As the frond becomes older, however, it is often 

 erect, for the hairs break away except at the base, and few new ones 

 are formed on the growing parts. The adult frond is somewhat rigid, 

 not as much so indeed as that of Padiua, for there is no incrustation 

 of calcic carbonate, as in the case of the latter. The surface of the 

 frond is marked with fine striations, running lengthwise along the frond, 

 which can be faintly seen with the naked eye and plainly with a hand 

 lens. Running in the opposite direction one can see, at considerable 

 intervals, very strongly marked concentric lines. In its histological 

 structure, Z. variegata resembles Z. parvula to a considerable extent, 

 difTering mainly in the number of layers of cells of which it is com- 

 posed. There is a row of the same brick-shaped marginal cells with 

 rounded apices as found m the other species (Fig. 1). These mar- 

 ginal cells are cut into two unequal parts by the formation of a series 

 of transverse walls. The larger and upper ones — if we consider the 

 frond in its erect position — continue their office of enlarging the 

 frond, while the lower ones become merged with the rest of the frond. 

 These lower cells soon divide mto three parts, by the formation of 

 two walls parallel to the surface of the frond in each cell, thus making 

 an inner and two outer layers. Later, more cells are cut off from the 

 central layer in the same direction as those flrst formed, until what may 

 be termed a cortex is formed, which may be from two to four cells in 

 thickness (Fig. 2). It must be borne in mind, however, that while 

 these layers thus formed will, to avoid confusion, be hereafter called 

 the cortex, one cannot well say where the cortical cells end and the 

 medullary cells begin, for as far as the color and character of the con- 

 tents go there is a gradual transition from one to the other. The color 

 of the contents of the outside layer is dark brown, that of the next 

 somewhat lighter, until in the medullary layer they are almost color- 

 less. The outermost Inyer not long after its formation divides up by 

 transverse and vertical walls into many small cells. The same di- 

 vision takes place in the next layer to a lesser extent, but the others 

 remain as they were when first formed. Thus in a fully developed 

 portion of the frond, beginning at the inside and going outwards, there 

 is first a large-celled central layer, then from one to two layers of 

 large flat cells, which, with the two outer small-celled layers, may be 

 said to constitute the cortex. In the young thallus the margin is 



