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-multiplication 

 accompanied 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 con- 

 siderably from one another in form and size. 



Nitella (Fig. 46, 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 

 1- 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, r/i) : the distal end is free. 



