INTRODUCTION. IV. 



THE FROND. 



Th.B frond or vegetable body of the compound Algse puts on a great variety of 

 shapes in different families, as it gradually rises from simpler to more complex 

 structures. In the less organized it consists of a string of cells arranged like the 

 beads of a necklace; and the cells of which such strings are composed may be 

 either globose or cylindrical. In the former case we have a moniliform string or 

 filament, and in the latter a filiform or cylindrical one. The term filament (in 

 Latin, yi^wm) is commonly applied to such simple strings of cells, but has occasionally 

 a wider acceptation, signifying any very slender, threadlike body, though formed 

 of more than one series of cells. This is a loose application of the term, and 

 ought to be avoided. By Kiitzing the term trichoma is substituted for the older 

 word filum, or filament. Where the filament for trichoma) consists of a single series 

 of consecutive cells, it appears like a jointed thread ; each individual cell consti- 

 tuting an articulation, and the walls between the cells forming dissepiments or nodes, 

 terms which are frequently employed in describing plants of this structure. 

 Where the filament is composed of more series of cells than one, it may be either 

 articulated or inarticulate. In the former case, the cells or articulations of the minor 

 filaments which compose the common filament are all of equal length ; their 

 dissepiments are therefore all on a level, and divide the compound body into a 

 series of nodes and internodes, or dissepiments and articulations. In the latter, the 

 cells of the minor filaments are of unequal length, so that no articulations are 

 obvious in the compound body. In Polysiphonia and Rhodomela may be seen 

 examples of such articulate and inarticulate filaments. 



By Kiitzing the term phycoma is applied to such compound stems ; and when the 

 phycoma becomes flattened or leaf-like, a new term, phylloma, is given to it by the 

 same author. These terms are sometimes convenient in describing particular struc- 

 tures, though not yet generally adopted. The cells of which compound stems 

 (or fhycomata), 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 often necessary, before we can ascertain the position of an individual plant whose 

 relations we Avish to learn. Sometimes all the cells have a longitudinal di- 

 rection, their longer axes being vertical. Very frequently, this longitudinal 

 arrangement is found only toward the centre of the stem, while toward the circum- 

 ference 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 perip)hery, or peripheric stratum, forming the outside layer or cir- 

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

 of which it is composed being very strongly and closely glued together ; in other 

 cases it is very lax, each individual filament lying apart from its fellow, the 

 interspaces being filled up with vegetable mucus or gelatine. This gelatine differs 

 greatly in consistence ; in some Algaj it is very thin and watery, in others it is 



