THE CELL aye 
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. 
