BOTANY. 



III. THE PRIMARY MERISTEM.* 



109. Under this name are grouped the unformed and 

 growing tissues found at the ends of young stems, leaves, and 

 roots. In these parts the tissues described above (paragraphs 

 99 to 108) have not yet formed ; they are, on the contrary, 

 composed entirely of a mass of thin-walled, growing, and 

 dividing cells containing an abundance of non-granular pro- 

 toplasm. In the lower plants the meristem-cells do not 

 change much in their configuration or general structure as 

 they develop into the ordinary plant-cells ; but the higher 

 the type of plant, the greater are the changes which take 

 place during the development of meristem into permanent 

 tissues. 



110. In most of the plants outside of the Phanerogams 

 the primary meristem is the result of the continually repeated 

 division of a single mother-cell situated at the apex of the 

 growing organ. In the simplest forms this apical cell is the 

 terminal one of a row of cells, as in many algae and fungi. 

 The apical cell, in such cases, keeps on growing in length, 

 and at the same time horizontal partitions are forming in its 

 proximal portion. In this way long lines of cells may 

 originate. 



In the more complicated cases the segments cut off from 

 the apical cell grow and subdivide in different planes, so as 

 to give rise to masses of cells. The partitions which succes- 

 sively divide the apical cell are sometimes perpendicular to 

 its axis, but more frequently they are oblique to it. In most 

 mosses, for example (Fig. 76), the apical cell is a triangular, 

 convex-based pyramid, whose apex is its proximal portion. 

 The successive segments are cut off from the apical cell by 

 alternate partitions parallel to its sides, thus giving rise to 

 three longitudinal rows of cells. Most Pteridophytes have 

 an apical cell not much different from that of the ma- 

 jority of mosses. In Equisetum, for example, it is an in- 

 verted triangular pyramid, having a convex base (Fig. 77 ; 



* From tlie Greek /if/wS, part, and Tt-pviev, to cut off. This tissue is 

 sometimes called Proto-meristem. 



