CELL-CONTENTS AND FORMS OF CELLS. 253 



it holds in solution are the only sources of the food supi)ly of 

 green plants, it follows that the higldy complex proteins trace 

 their origin to these comparatively simple substances. By some 

 it is supposed that the final stages in the building up or synthesis 

 of the proteins take place in the leaves, but it is probable that they 

 take place in all the growing parts of the plant. It has already 

 been stated in the paragraph on proteins that seeds contain re- 

 serve materials which are broken up into simpler compounds 

 through the action of certain enzymes, and thus made available 

 for the seedling. It is claimed that these compounds are prin- 

 cipally amino acids, and that of these aspartic and glutaminic acids 

 occur in largest amount and that these two acids are found in 

 different relative amounts in different plants. It is furthermore 

 claimed by some authors that by certain syntheses these com- 

 pounds are respectively converted into asparagin and glutamin, 

 both of which occur as reserve materials in seeds and in other 

 parts of plants as well. Yet other syntheses take place w^hereby 

 asparagin and similar bodies are converted into albumin and other 

 proteins. In the Coniferae the part played by asparagin and 

 glutamin in protein syntheses is taken by arginin, which substance 

 is found in considerable amount in the seeds of the plants of this 

 group. 



Growing Points and Meristems. — Plants are distinguished, 

 for the most part, by having distinct growing points, known as 

 vegetative points. These occur at the apex of shoots and roots and 

 at definite lateral points, being in the stem near the surface and 

 in roots beneath the endodermis. The walls of the cells in these 

 regions are very thin and consist almost entirely of cellulose. 

 The cells are compactly arranged ^nd are more or less polygonal 

 or somewhat elongated. They are rich in protoplasm, capable of 

 rapid division, and constitute the tissue known as primary meri- 

 STEM. In the root three kinds of primary meristem (Fig. 132) 

 are distinguished: (i) The plerome (m, f, g), an axial meri- 

 stem, which gives rise to the central cylinder or stele; (2) the 

 periblem (x, r), or meristematic tissue, which gives rise to the 

 primary cortex; and (3) the dermatogen (e), from which the 

 epidermis is developed. In addition to these three meristematic 

 zones there is at the apex, lying next to the dermatogen, a mcri- 



