LESSON XXI 



NITELLA 



IN the linear, superficial, and solid aggregates discussed in 

 the three previous lessons, the organism was seen to be 

 composed of cells which in most cases differed but little 

 from one another, all complications of structure being due 

 to a continued repetition of the process of cell-multiplica- 

 tion accompanied, except in Laminaria and its allies, by 

 little or no cell-differentiation. In the present lesson we 

 shall make a detailed study of a solid aggregate in which 

 the constituent cells differ very considerably from one 

 another in form and size. 



Nitella (Fig. 45, A) is a not uncommon fresh-water weed, 

 found in ponds and water-races, and distinguished at once 

 from such low Algae as Vaucheria and Spirogyra by its ex- 

 ternal resemblance to one of the higher plants, since it 

 presents structures which may be distinguished as stem, 

 branches, leaves, &c. 



A Nitella plant consists of a slender cylindrical stem, 

 some 15-20 cm. and upwards in length, but not more than 

 about J mm. in diameter. The proximal end is loosely 

 rooted to the mud at the bottom of the stream or pond by 

 delicate root filaments or rhizoids (A, rh) : the distal end is 



