8 PROTOPLASM AND PLANT CELLS 



cells are binucleate while they are uninucleate in the 

 remaining stages. In some groups of plants, however, 

 we find that, enclosed in an outer cell wall, there is a 

 mass of cytoplasm containing many nuclei. Such a 

 structure is called a coenocyte. It is frequently re- 

 garded as consisting of as many cells as nuclei are present, 

 not separated, however, by partition walls. Perhaps it 

 may better be considered as a sort of compound cell as 

 the nuclei do not seem to control definite masses of cy- 

 toplasm. In some coenocytes of the seaweed Grifnthsia 

 over 4,000 nuclei are present, while in the enormous 

 coenocyte of Caulerpa, likewise a seaweed, which often 

 attains a length of several decimeters, the number of 

 nuclei is vastly greater. Coenocytes are mostly re- 

 stricted to certain groups of lower plants, but cells of 

 coenocytic nature may occur even in the higher plants. 



17. In shape cells are very variable. Usually we find 

 that free-living cells approach the spherical shape al- 

 though they are often elongated somewhat. Cells 

 united to other cells are usually flattened on the sides 

 where they are in contact. When surrounded by cells 

 at all sides cells are usually more or less regular, several 

 to many-sided polyhedra. Some cells are cylindrical 

 while often we have fiber or spindle shaped cells. Some 

 cells are lobed or branched. 



Laboratory Studies. It is assumed that the attempt will 

 not be made to use this book without endeavoring to carry 

 out in the laboratory all or at least a selection of the laboratory 

 exercises suggested here and there in connection with the 

 various topics. So far as possible the suggested exercises 

 have been made simple enough for the student to undertake 

 himself, depending as little as possible upon specimens prepared 

 or experiments set up by the teacher. It is absolutely essential 

 that each student have the use of a good compound micro- 

 scope, and that he possess the proper tools for making sections, 



