CHAPTER VL 



TISSUES. 

 § I. The Vaeious Aggregations of Cells. 



lu the organisms which compose the vegetable kingdom 

 cells are found principally under the following conditions of 

 aggregation : 



92. — (1.) Single Cells. A large number of the lower 

 plants, during all or a considerable part of their existence, 

 are composed of single cells. They may be round, as in 

 Saccharomyces and Protococcus, or elongated or even filiform, 

 as in certain Bacteria. It is only in the lowest groups that 



Fig 49.— Pedtaslrum gramdatum. A. the young cells in their motile state, en- 

 CIOBea m the membrane of the mother-cell. B, the young cells beginning to arrange 

 themselves in a cell-family. C, the cell-family fully developed.— A f ter Braun. 



adult plants are composed of single cells, but it is an 

 embryonic condition of all others. 



93. — (3.) Families, or Spurious Tissues. There are 

 some oases in which cells which are at first distinct after- 

 wards become united more or less closely into a common 

 mass, -which may be denominated a Cell-Family, or Spurious 

 Tissue. 



{a) Pediastrum and Hydrodictyon furnish the best examples of true 



