26 INTRODUCTION 



brackish ditches and on the stones and woodwork of wharves are 

 also species of the lowest orders of algae 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 Confervacece, form a large 

 part of the green vegetation between tide-marks. 



The vegetative body of a thallophyte is a tkallus, and corre- 

 sponds to stem and leaf. It is also called a frond. What 

 corresponds to the root of flowering plants is in algas 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 algae 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 

 algae other than the name implies, whereas real roots absorb the 

 nourishment upon which plants live. Algae are nourished by the 

 substances held in solution by the water which surrounds them. 



Algae 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 algae 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 CladopJwra. 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. 



