zx8 THE STRUCTURE OF ECONOMIC PLANTS 



During the first week or ten days of seedling development, while 

 the hypocotyl is obtaining its maximum length of 5 to 6 cm., the 

 epicotyl elongates rather slowly. Later, the rate of epicotyle- 

 donary growth is accelerated; and, according to Berkman (3), the 

 first internode reaches its normal length of 3 to 5 cm. by the twelfth 

 or thirteenth day. The first pair of foliage leaves above the cotyle- 

 dons are simple and ovate with serrate margins, and are functional 

 for a short time only. In most instances, the second and third 

 pairs are digitate, the former usually having three leaflets and the 



latter four or five. The leaf- 



Is 



■© 



ff?) 



\ 



Fig 



109. 



lets above the third pair are 

 as described under general 

 morphology. 



The Primary Root. — The 

 root is diarch, with two 

 groups of primary phloem 

 flanking the primary xylem 

 strand. The protoxylem abuts 

 the pericycle, which is multi- 

 seriate, consisting of four or 

 more layers lying centrad to 

 the endodermis. In the inter- 

 cotyledonary plane, it is usu- 

 ally only two layers in width. 

 The endodermis is uniseriate 

 and Casparian strips begin to 

 differentiate at about the time that the metaxylem matures, but 

 they do not become well developed until secondary thickening is 

 initiated. The cortex is composed of five to seven concentric lay- 

 ers of parenchymatous cells with prominent intercellular spaces. 

 The outermost layer forms a distinct hypodermis which is similar 

 in structure to the epidermis except that it produces no root hairs. 

 (Fig. no, A.^ The root has a blunt cone-shaped root cap. 



There are several accounts of the structure and ontogeny of the 

 root, and these do not agree in all respects. Van Tieghem and 

 Douliot (i8) described the pericycle as a single layer of cells, and 

 also interpreted the periblem as one layer of initials. Flahault 

 (14), Briosi and Tognini (7), and Berkman (3) report the periblem 

 as consisting of two layers of initials, and the pericyclic layer as 

 soon becoming multiseriate as a result of periclinal divisions. 



The fruit and stages in development 

 of seedling. 



