STRUCTURE OF PLANTS. 475 



as the fundamental unit for convenience' sake, and because it presents 

 a definite form ; it is not, however, to be regarded as the ultimate 

 structural unit, because detached fragments of it are capable of in- 

 dependent existence under certain circumstances, and the proto- 

 plasm, in which in all cases the whole vital activity centres, is 

 capable of living and moving without a cell-wall. 



Cell-contents. The cell is the elementary organ of vegetable 

 structure ; but it is not the smallest or most simple definite form in 

 which organic matter may exist in plants. In the contents of cells 

 we find granules of various kinds &c., and also fibres ; the former, 

 however, are not direct constituents of tissues, but occur only 

 among the contents of cells, as more or less transitory conditions of 

 assimilated matter; while the latter merely form parts of the 

 structure of the cell-membrane. 



Uni- and Multicellular Plants. Plants of the lowest organiza- 

 tion consist of the ultimate or elementary organs in their simplest 

 forms, and may even be so simple as to consist of a single ele- 

 mentary organ or cell (figs. 503 E, 513). A step higher, we find 

 plants composed of a few cells connected together into a definitely 

 arranged group in their earlier period of existence, and subse- 

 quently separating entirely into the constituent cells, each of 

 which lays the foundation of a new colony. 



Tissues. By far the greater part of the species of plants are 

 composed of an indefinite number of cells permanently combined 

 together and forming what are termed the tissues. If the cells entering 

 into the composition of a tissue are essentially alike, they form a 

 simple tissue ; if cells which have undergone modifications which 

 give them an essentially diverse character are combined in an ana- 

 tomically well-defined tissue, this is called a compound tissue. 



In the Algae, especially the simpler membranous or filamentous forms, 

 we may readily see the uniformity of the character of the cells throughout 

 the thallus (p. 435) ; the same uniformity prevails through the cells of 

 such tissues as the pith of Dicotyledonous stems, &c. But if we examine 

 the wood surrounding this pith, or even the ribs running into the leaves, 

 we find a variety of conditions of the elementary organs within the well- 

 defined limits of these portions of woody tissue. 



Cellular tissue. The simple tissues of plants are divisible again 

 into two primary groups, according to the mode of union of the 

 constituent cells. In proper Cellular Tissues the cells, however 

 firmly coherent, are only in contact by their walls, which form a 

 persistent boundary between them. In a series of tissues most 

 extensively developed in plants of high organization, the cells enter 

 into closer relation, becoming confluent by the absorption of their 

 contiguous surfaces, and thus converted into more or less extensive 



