254 A TEXT-BOOK OF BOTANY. 



stematic group of cells which give rise to the root-cap, known as 



the CALYTROGEN (s). 



At the growing point of the stem three meristematic zones are 

 also distinguished, namely, plerome, periblem, and dermatogen. 

 They are not, however, so well marked as in the case of roots. 



The tissues which are developed from the primary meristems 

 constitute the primary structure. With the growth in thickness 

 of the stems and roots of Dicotyledons other meristematic cells, 

 known as secondary meristems, arise. These are of two kinds : 

 ( I ) One wdiich gives rise to the xylem and phloem, known as the 

 CAMBIUM, and (2) another which gives rise to the cork, known as 

 phellogen. The tissues formed from the secondary meristems 

 constitute the secondary structure of older dicotyledonous 

 stems and roots. 



While the point of vegetation in the higher plants (spermo- 

 phytes) embraces a number of cells, in the lower plants the tissues 

 can be traced back to a single apical cell. 



Cell- Wall. — Origin and Composition.-:— It is formed by the 

 protoplasm, and varies in composition at different stages of the 

 growth of the cell, and according to the various functions it has 

 to perform. 



In order to thoroughly understand the nature and composi- 

 tion of the cell-wall, it is necessary to study the origin and forma- 

 tion of new cells. Growth of the plant is attended not only by 

 an increase in the size of the cells, but by their division (Fig. 85) 

 new cells are also formed. Cells that have the property to divide 

 and form new cells are known as meristematic cells and constitute 

 the meristem. The new and dividing walls resulting from the 

 division of the cells consist of a number of substances. When a 

 cell divides, the two daughter protoplasts which result from the 

 division of the nucleus and cytoplasm are separated by the forma- 

 tion of a new wall between them (Fig. 85). The first layer 

 formed is apparently different from the subsequent layers and is 

 known as the middle plate or middle lamella. This layer is 

 soluble in, or readily attacked by, solutions of the alkalies or solu- 

 tions containing free chlorine. It is insoluble in sulphuric acid, 

 and readily stained by the aniline dyes. While usually more or 

 less permanent, this middle plate may be finally absorbed, as in 



