THE SMITHSONIAN INSTITUTION. 95 



and when the pliycoma "becomes flattened or loaf-like, a new term, 

 phylloma, is given' to it by the same author. These terms are some- 

 times convenient in describing particular structures, though not yet 

 generally adopted. Tiie cells of which compound stems (or pliyco- 

 mata) are composed are very variously arranged, and on this cellular 

 arrangement, or internal structure of the stem, depends frequently 

 the place in the system to which the plant is to be referred. A close 

 examination, therefore, of the interior of the frond, by means of thin 

 slices under high powers of the microscope, is olten necessary, before 

 we can ascertain the position of an individual plant whose relations 

 we wish to learn. Sometimes all the cells have a longitudinal direc- 

 tion, their longer axes being vertical. Very frequently, this longi- 

 tudinal arrangement is found only towards the centre of the stem, 

 while towards the circumference the cells stand at right angles to 

 those of the centre, or have a horizontal direction. In such stems we 

 distinguish a proper axis, running through the frond, and a periphery, 

 ox peripheric stratum, forming the outside layer or circumference. 

 Sometimes the axis is the densest portion of the frond, the filaments 

 of which it is composed being very strongly and closely glued to- 

 gether ; in other cases it is very lax, each individual fihiment lying 

 apart from its fellow, the interspaces being filled up with vegetable 

 mucus or gelatine. This gelatine differs greatly in consistence ; in 

 some Algte it is very thin and watery, in others it is slimy, and in 

 others it has nearly the firmness of cartilage. On the degree of its 

 compactness and abundance depends the relative substance of the 

 plant ; which is membranaceous where the gelatine is in small quan- 

 tity; gelatinous where it is very abundant and somewhat fluid ; or 

 cartilaginous where it is firm. 



The frond may be either cylindrical or stem-like, or more or less 

 compressed and flattened. Often a cylindrical stem bears branches 

 which widen upwards, and terminate in leaf-like expansions, which 

 are of various degrees of perfection in different kinds. Thus some- 

 times the leaf, or phyUorna, is a mere dilatation ; in other cases it is 

 traversed by a midrib, and in the most perfect kinds lateral nervelets 

 issue from the midrib and extend to the margin. These leaves are 

 either vertical, which is their normal condition, or else they are in- 

 clined at various angles to the stem or axis, chiefly from a twisting 

 in their lamina, the insertion of the leaf preserving its vertical posi- 

 tion. They are variously lobed or cloven, and in a few cases (as in 

 the Sea Colander of the American coast) they are reguhirly pierced, 

 at all ages, with a series of holes which seem to originate in some 

 portions of the lamina developing new cells with greater rapidity than 

 other parts, thus causing an unequal tension in various portions of the 

 frond, and consequently the production of holes in those places where 

 the growth is defective. Such plants, tliough they form hice-like 

 fronds, are scarcely to be considered as net-works. Net-like fr(mds 

 are, however, formed by several Alg;e where the branches regularly 

 anastomose one with another, and form meshes like those of a net. 

 Most species with this structure are peculiar to the Southern Ocean, 

 but in the waters of the Caribbean Sea are found two or three which 

 may perhaps yet be detected on the shores of the Florida Keys. In 



