212 



GENERAL MORPHOLOGY OF PLANTS 



pendently of other plants. They obtain their mineral and nitrog- 

 enous foods from the water, while they fix the carbon from the 

 carbonic acid absorbed from the water, in the presence of the 

 chlorophyll and sunlight. 



The characters of structure and reproduction should be studied 

 in connection with the individual plants. 



GREEN ALG.E* (CHLOROPHYCE^E). 



The Conjugating Green Algce (Conjugates}. 



349. Spirogyra. The plant spirogyra lives in fresh water in 

 ponds, the borders of lakes, or in pools. 

 Sometimes it is found in very slow-running 

 water. It is in the form of simple threads 

 or filaments which may be quite long and 

 are unbranched. Large numbers of these 

 threads are tangled together into a mat 

 which floats in the water. Much gas which 

 is given off during photosynthesis is caught 

 in the meshes of the tangle, buoys the mat 

 of the alga up to the surface of the water 

 where the light is more efficient, and gives 

 the plant a frothy appearance, which sug- 

 gested such names as " pond scum," " frog 

 spittle," etc. The threads are made up of 

 cells which are cylindrical in form and 



Fig. 169. Fig. 170. * To THE TEACHER. The number of green 



Spirogyra . Spirogyra j studied must be determined by the teacher, 

 before plac- in 5 per cent a o ' 



ing in salt salt solution. an d will depend to some extent on the time, the 



facilities, and material at hand. If only one is 



studied carefully it preferably should be Spirogyra. If two, then Vaucheria 

 or (Edogonium should be included because of the differentiation of the sex 

 organs. In more advanced classes the shield Coleochaete might be included. 

 In addition to laboratory work, such portion of the text should be studied* 

 as the teacher finds time and adaptability of the pupils will permit. The 

 general features of the plant body, the progression from single cells to threads 

 and cell plates, the general features of sex organs and their differentiation 

 into two kinds, as well as the life cycle, should always be kept foremost. 



