CELLULAR STRUCTURE OF ORGANISMS 



93 



protos = first + zoon = animal), although they are not strictly 

 unicellular. 



The same principle is illustrated by many of the lower plants, 

 of which a single example will be given. 



Ulothrix. One of the common fresh-water pond scums, 

 found everywhere in ditches by the roadside, is made of a 

 green plant, Ulothrix; Fig. 41. Ulothrix consists of a long, 

 slender thread formed by a row of nearly cylindrical cells, 

 placed end to end; Fig. 41 A. The individual threads are 

 barely visible to the naked eye. In each one of these cells 

 rnay be seen green coloring matter, chlorophyll (Gr. chloros = 

 green + phyllon = leaf), and each cell contains a nucleus. 

 The cells are identical from one end of the thread to the other, 

 differing only slightly in 

 size, and each of them is 

 capable of carrying on 

 all the functions of life 

 independently. 



The reproduction 

 in Ulothrix is interest- 

 ing; and, like some or- 

 ganisms already studied, 

 it shows two quite 

 distinct methods. The 

 first and simplest is as 

 follows: The contents of 

 one of the cells breaks up 

 into several parts, which, 

 after a time, escape upon 

 the bursting of the 

 plant's cell wall. As they 

 come out, each is seen to 

 be provided with four little flagella and is thus enabled to swim. 

 They are called zoospores (Gr. zoon = animal) ; Fig. 41 a. After 

 swimming for a time they settle down, lose their cilia, and 



FIG. 41. PLANTS MADE UP OF COLONIES 

 OF SINGLE CELLS 



A, Ulothrix. a, shows the process of multiplica- 

 tion by the formation of zoospores; b to /, show the 

 formation of sex cells, their conjugation with each 

 other; g, their subsequent division into spores; 

 h, a single spore which grows into a new thread, like 

 the original shown at large A. B, Pediastrium. 



