26 INTRODUCTION 
brackish ditches and on the stones and woodwork of wharves are 
also species of the lowest orders of algze and increase by cell- 
division. Many of them are in colonies incased in gelatinous 
matter. These, together with plants of a little higher order, 
though still of low organization, the Confervacec, form a large 
part of the green vegetation between tide-marks. 
The vegetative body of a thallophyte is a thallus, and corre- 
sponds to stem and leaf. It is also called a frond. What 
corresponds to the root of flowering plants is in alge a disk or 
conical expansion of the base of the plant. It is simply a hold- 
fast by which the frond attaches itself to any submerged material. 
The algw which grow on sandy shores and on corals have hold- 
fasts which branch like fibrous roots and penetrate porous sub- 
stances in all directions; but this is only for greater stability, 
and is an adaptation to the habitat. Holdfasts do nothing for 
alge other than the name implies, whereas real roots absorb the 
nourishment upon which plants live. Algee are nourished by the 
substances held in solution by the water which surrounds them. 
Algee are the lowest and simplest in organization of all plants, 
because they are composed of but one class of cells, such as in 
flowering plants are called the parenchyma, or soft cells, these 
being the ones which compose the pulp of the leaf. In the lowest 
orders of alge single cells constitute individual plants, as in 
Pleurococcus; but in the higher forms, such as Sargassum, they 
arrange themselves in such a variety of combinations as to re- 
semble plants which have leaf and stem. The botanical distinc- 
tion is that in leaf and stem there would exist the woody and the 
vascular cells as well as the parenchyma cells. 
Beginning with plants composed of a single cell, the next 
development is into filamentous plants, which are single thread- 
like rows of cells, as in Cladophora. In Ulva is seen the earliest 
type of an expanded leaf. The cells are here arranged in a hori- 
zontal surface of plate-like or ribbon-like shape. 
In Ulva there is a double layer of cells. The layers separate 
in Enteromorpha, giving a hollow or tubular form. In Monos- 
troma a double layer is opened or torn apart, giving a frond with 
a single layer of cells. 
