LESS. XX EXTERNAL CHARACTERS 205 



ment {seg) consisting of a proximal internode (/;//. mi) and distal node 

 {nd) : the leaves (/) arranged in whorls and ending in leaflets (/) : the 

 rhizoids (;//) : and two branches {br), each springing from the axil of a 

 leaf and ending, like the main stem, in a terminal bud {term. bud). 



B, distal end of a shoot with gonads attached to the leaves : ovy, the 

 ovaries ; spy, the spermaries. 



C, distal end of a rhizoid. 



D, distal end of a leaf (/) with two leaflets (/), showing the chroina- 

 tophores and the white line. The arrows indicate the direction of rota- 

 tion of the protoplasm. 



E, distal end of a leaflet, showing the general structure of a typical 

 cell of Nitella in optical section : c. w, the cell-wall ; p/sm^, the quies- 

 cent outer layer of protoplasm containing chromatophores (chr) ; plsm'^, 

 the inner layer, rotating in the direction indicated by the arrows, and 

 containing nuclei {nu) ; vac, the large vacuole. 



F, terminal bud, partly dissected, showing the nodes (tid), internodes 

 {int. fid), and leaf-whorls (/), numbered from I to 4, starting from the 

 proximal end ; gr. pt, growing point. 



G, distal end of a leaf (/) with two leaflets {!), at the base of which 

 are attached a spermary {spy) and two ovaries {ovy). 



from such low Algge 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 \ 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 

 free. Springing from it at intervals are circlets or whorls of 

 delicate, pointed leaves {I). 



Owing to the regular arrangement of the leaves the stem 

 is divisible into successive sections or segments {seg), each 

 consisting of a very short distal division or node {nd) from 

 which the leaves spring, and of an elongated proximal 

 division or internode {int. nd), which bears no leaves. 



Throughout the greater part of the stem the whorls of 

 leaves are disposed at approximately equal distances from 

 one another, so that the internodes are of equal length, bi!t 



