PROTOPHYTA 



59 



Fig. 30— CLOSTERIUM 



One of the largest of the desmids. The nucleus, N, 



indicates the meeting place of the two valves. A number 



of pyrenoids, P, sometimes called starch-forming nuclei, 



are arranged symmetrically 



From a photomicrograph by the author 

 Magnification, 160 



bilaterally symmetrical.* In the majority of species of this group 

 a central constriction called "the isthmus" divides the organism 

 into two equal parts, 

 each half being termed 

 a semicell. In the genus 

 Closterium (Fig. 30) a 

 constriction of the cell 

 body is present but the 

 cell wall remains in- 

 tact, while in a few 

 genera, e.g., many spe- 

 cies of Penium, there is 

 no evidence whatsoever 

 of a constriction. The 

 genus Cosmarium is the 

 largest in number of species, many of them having exquisitely 

 ornamented cell walls showing symmetrical patterns in the ar- 

 rangement of granules, and of the papillae or knobs of various 



forms and sizes (Fig. 31 — 

 Xanthidium). The great- 

 est diversity of forms in 

 any one genus is found in 

 Staurastrum, common in 

 reservoirs and lakes, while 

 the most spectacular forms 

 in respect to minute sym- 

 metry are certain species of 

 Micrasterias. The outer 

 wall in all genera of this 

 group consists of two 

 valves, one for each semi- 

 cell. One of these fits into 

 the other like a pill-box 

 into its cover. 

 Magnification, 480 Reproduction of these 



plants occurs by a peculiar process of budding, a type of divi- 

 sion which in the simpler desmids consists in the formation 



* Bilateral symmetry is the condition of having the right and left sides of the 

 body similar; the majority of metazoa exhibit bilateral symmetry in their external 

 forms. 



Courtesy of the American Museum of 



Natural History 



Fig. 31— A MICROSCOPIC WATER 

 PLANT 



Xanthidium armatum is composed of two 



connected cells armed with spines, which 



enable it to cling to plant stems 



