GREEN ALG& 239 



buckle joint connecting two adjacent cells of the same thread. 

 The zygospore is formed in one of the two conjugating cells 

 by the fusion of the contents of the two cells into one, the 

 shrinkage of this mass from the wall forming a rounded body 

 with a thick wall within the female gamete case, or egg case. 

 The fusion of the two nuclei in the zygospore completes the proc- 

 ess of fertilization of the egg which passes through a resting stage. 

 The life history is sometimes spoken of as a life cycle. It may be 

 represented by the diagram or by the following formula:* 



/male gamete \ 



Plant, veg. (, , / zygospore = fertilized egg Plant, 



\female gamete/ 



etc. 



This formula may be abbreviated by the use of signs as follows: 



P P P <^'\E P, etc., in which P stands for plant and 

 \g/ 



its repetition indicates multiplication of the plant from similar 

 parts, not by means of special reproductive or propagative 

 bodies; g= gamete, E = the fertilized egg, zygospore or oospore 

 as it is called in special cases. 



371. Zygnema. Zygnema is another genus of filamentous 

 algae closely related to Spirogyra. It differs chiefly in the form 

 of the chlorophyll bodies. These are star-shaped and two in a 

 single cell. The nucleus in the cell is not so conspicuous. 

 Growth, multiplication and conjugation take place as in Spiro- 

 gyra. The life history follows the same course and the plant is 

 found under, similar conditions. 



372. The desmids. The desmids form another family of 

 conjugating algae. They are mostly single-celled plants of very 

 beautiful form and markings, but a few of them unite into fila- 

 ments. They occur in fresh water, often mingled with spiro- 

 gyra and other green algae. The single-celled forms are bisym- 

 metrical, each half being exactly like the other. They multiply 

 by dividing so as to separate these two halves, and each half then 



* This formula, and those which follow other groups of plants, are not to be 

 memorized by the pupil. It is simply presented to serve as a graphic repre- 

 sentation of the life cycle. 



