18 PLANT-LIFE ON LAND [ch. 



flexible, so that when left by the receding tide it 

 adheres to the substratum, but floats off" again as 

 the Avater rises, yielding to every motion, but re- 

 maining fixed at the base during life (Fig. 1, i). 

 Sections show that the flat expansion is composed 

 of two layers of oval cells, which are all alike, 

 having a protoplast containing a nucleus and green 

 chlorophyll-body, and surrounded by a swollen cell- 

 wall. But towards the base of the plant some of 

 the cells may be elongated downwards into tubes, 

 which force their way between the two layers, and 

 reaching the base add to the strength of the 

 attachment-disc (Fig. 1, 2, 3). Thus constructed the 

 plant may grow to a large size, though still pre- 

 serving its simple outline. 



There are other genera of Green Algae, diftei-ing in 

 their form, but resembling Ulva in all essential points, 

 such as the common genus Enteromorpha with its 

 tubular body, gut-shaped as its name implies, produced 

 by the separation of its two layers which correspond 

 to those of Ulva ; or Monostroma, in which at an 

 early stage the tube is ruptured, so that a broad 

 green expansion is produced, only one layer of cells 

 in thickness. But all such forms, including Ulva 

 itself, are held to be merely consequences of the 

 elaboration of the simple filament of cells, such as 

 is seen in the genus Ulothrix, various species of 

 which inhabit fresh or salt water (Fig. 2, A). If the 



