PRINCIPAL GROUPS OF PLANTS. 17 



ing most of the Green Algae, and the Marine Algae or Sea-weeds, 

 including most of the brown and red forms. 



Algae are classified in three natural groups, not only on account 

 of color differences, but because of certain corresponding struct- 

 ural relationships, thus : 



Chlorophyceae (Green Algae). 

 Phaeophyceae (Brown Algae). 

 Rhodophyceae (Red Algae). 



Arranging the Algae in this way provides the simplest classi- 

 fication. But in addition to these groups there is another some 

 what isolated group that will be taken up first before the Chloro- 

 phyceae, — namely, the Conjugatae. These are Green AlgcC con- 

 sisting of either single cells or a chain of cells united into threads 

 and further characterized by dividing always in the one direction 

 so that a filament results. They furthermore do not produce 

 zoospores, but produce zygospores as a result of a union of two 

 'similar or only slightly dififerent cells. After a period of rest 

 they break from the outer membrane and develop directly into new 

 vegetable cells. To this class the Desmids and Spirogyra belong. 



The Desmids are unicellular Algae, varying from torpedo- 

 shaped to variously branched forms, occurring even in chains. 

 The protoplast is usually separated at or near the middle, where 

 the nucleus is located, dividing the cell into two symmetrical por- 

 tions (Fig. S, E). In the protoplast is a more or less complex 

 chromatophore, through the center of which are distributed a 

 number of globular pyrenoids. The latter are distinct structures 

 embedded in the chromatophores of Green Algae and consist of 

 a central protein substance surrounded by a number of starch 

 grains, and, therefore, give a purple reaction with iodine. Owing 

 to the fact that the Desmids are motile they were formerly con- 

 sidered to be members of the animal kingdom. The movement is 

 slow and steady and largely influenced by the light. There is also a 

 circulatory movement frequently observed in the contents of 

 active living material. In addition, there is almost always observ- 

 able at the ends a well-defined spherical vacuole containing 

 numerous small crystals of calcium sulphate which exhibit a 

 dancing movement due to surface tension and is known as molec- 

 2 



