8 
INTRODUCTION, 
IY. 
THE FROND. 
The frond or vegetable body of the compound Alga; puts on a great variety of 
shapes 'n 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, filum) 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 Kutzing 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 Kutzing the term p>hycoma is applied to such compound stems ; and when the 
phycoma becomes flattened or leafdike, 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 phycomata), 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 wish 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 periphery , 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 Algos it is very thin and watery, in others it is 
