144 FUNDAMENTALS OF STRUCTURE AND FUNCTION 



of the spindle. The centriole in many daughter cells divides imme- 

 diately into two, although in some cells it remains as a single body 

 until a new mitosis begins. 



Tissues 



Cells form aggregates called tissues, examples of which may be seen 

 in the woody cells making up the greater part of the stem of a plant ; 

 the elongated cells in this same stem which form the conducting 

 tissue; the flat protective cells covering the outside of the leaf, called 

 collectively the epidermis; and the large columnar cells filled with 

 green chloroplasts that form the parenchyma layer directly under the 

 epidermal cells. In our own body, we find numerous examples of famil- 

 iar tissues set apart for doing some particular work, such as the epithe- 

 lial, or protective, tissues ; the connective tissues, which serve to bind 

 the various groups of cells together ; the muscular tissues, of several 

 kinds ; the supporting tissue cells, which help to build the bones ; 

 glandular tissues ; the nervous tissues of several kinds ; and the blood, 

 which, though fluid, yet contains cells, and is classed as a circulating 

 tissue. 



The Tissues in Plants 



It is a difficult matter to make a classification of tissues that will 

 fit all plants and yet be simple enough to use at this stage of our 

 biological knowledge. But the following will give us a general survey 

 which can later be expanded by the student of botany. 



The Meristematic Tissues. These cells in general are small, 

 thin walled, and rich in protoplasm. They are found in the rapidly 

 growing parts of plants, the buds, the tips of the roots, and in growing 

 layers. They represent the primitive and embryonic tissues. 



The Protective Tissues. Such are the epidermal cells covering 

 leaves. These are often waterproofed with a waxy material called 

 cutin. Such layers are found on the outside of the stem, root, and 

 even the fruit, forming a protective covering. In the stem and the 

 root, the epidermis is often replaced by a layer of corky cells, while on 

 leaves, stems, and flowers the epidermal cells frequently develop 

 hairs or scales, which sometimes secrete sticky substances. 



The Fundamental Tissues. These groups of cells form the great 

 mass of plant tissue, such as the soft green parts of the leaf, the pith 

 or cortex of plant stems, the soft parts of flowers and fruits. These 

 cells differ greatly in size and shape in different parts of the plant, 



