THE CELL 57 



ORIGIN OF MULTICELLULAR PLANTS 



All multicellular plants are built up by the repeated cell 

 division of one original cell. If the cells formed are similar in 

 structure and function, they form a tissue. In multicellular 

 plants many different kinds of tissues will be formed as a result 

 of cell division, since there are many different functions to be 

 performed by such an organism. When several of these tissues 

 become associated and their functions are correlated, they form 

 an organ. The association of several organs in one form makes 

 an organism. The oak-tree is an organism. It is made up of 

 organs known as flowers, leaves, stems, roots, etc. Each of 

 these organs is in turn made up of several kinds of tissue. In 

 some cases it is difficult to designate a single function to an 

 aggregation of cells (tissue). In fact, a tissue may perform 

 different functions at different periods of its existence or it 

 may perform two functions at one and the same time; as an 

 example, stone cells, whose primary function is mechanical, in 

 many cases function as storage tissue. The cells forming the 

 tissues of the plant, in fact, show great adaptability in regard 

 to the function which they perform. Nevertheless there is a 

 predominating function which all tissues perform, and the 

 structure of the cells forming such tissues is so uniform that it 

 is possible to classify them. 



The functional classification of tissues is chosen for the 

 purpose of demonstrating the adaptation of cell structure to 

 cell function. If the cells performing a similar function in the 

 different plants were identical in number, distribution, form, 

 color, size, structure, and cell contents, there would not be a 

 science of histology upon which the art of microscopic pharma- 

 cognosy is based. It may be said, however, with certainty, 

 that the cells forming certain of the tissues of any given species 

 of plant will differ in a recognizable degree from cells perform- 

 ing a similar function in other species of plants. Often a tissue 

 is present in one plant but absent in another. For example, 

 many aquatic plants are devoid of mechanical fibrous cells. 

 The barks of certain plants have characteristic stone cells, while 

 in many other barks no stone cells occur. Many leaves have 

 characteristic trichomes; others are free from trichomes, etc. 



