232 



GENERAL MORPHOLOGY OF PLANTS 



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^*(CHLOROPHYCE^E). 



The Conjugating Green Algce (Conjugate). 



Spirogyra. The plant spirogyra lives in fresh water in 

 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 photo- 

 synthesis 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 ap- 

 pearance, which suggested such names as 

 "pond scum," "frog spittle," etc. The 

 threads are made up of cells which are 

 cylindrical in form and joined end to end. 



Fig. 178. 



Spirogyra 

 before plac- 

 ing in salt 

 solution. 



Fig. 179. 



Spirogyra 

 in 5 per cent 

 salt solution. 



* To THE TEACHER. The number of green algae 

 studied must be determined by the teacher, and 

 will depend to some extent on the time, the facili- 

 ties, 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. 



