344 THE STRUCTURE OF ECONOMIC PLANTS 



ening and wrinkling of the seed coat, followed by a rapid swelling 

 of the cotyledons, and a penetration of the seed coat at the micro- 

 pylar zone by the tip of the conical hypocotyl. The short hypo- 

 cotyledonary portion of the axis is, for the most part, root-like; 

 and a few millimeters beneath the cotyledonary node, the epidermis 

 is of the root type producing root hairs. In some cases, the epi- 

 dermal cells of the first and second internodes are elongated so that 

 they resemble root hairs in appearance. 



The cotyledons are diverged at right angles to the seedling axis at 

 an angle of about 11.0° from each other. Throughout germination, 

 they remain in the testa; but the cotyledonary petioles elongate 

 sufficiently to draw the epicotyledonary portion of the axis out 

 from its original position between the cotyledons. The epicotyl, 

 which has differentiated the first leaves, or trifid bracts, prior to 

 germination, pushes up through the soil, and the differentiation 

 of the true foliage leaves follows. The stem is quadrangular with 

 comparatively short internodes, and the phyllotaxy is distichous. 

 The main shoot may die after a short period of growth, and the 

 development of the plant is continued by branches arising from buds 

 in the axils of the lower bracts or leaves. There are buds in the 

 axils of the cotyledons which usually remain dormant; but, when 

 the epicotyl is severed just above the cotyledons, as is done in 

 connection with some of the experimental work on growth hor- 

 mones, these may develop rapidly into lateral shoots. 



The Primary Root. — The ontogeny of the primary root corre- 

 sponds to the fourth angiospermous type as described by Janczewski 

 (17), in which growth is accomplished by means of a general 

 meristem that forms a transverse zone extending across the apex of 

 the root. (Fig. 17.) From this generative zone, cell divisions 

 on its distal surface produce the successive layers which form the 

 conical central portion of the calyptra, and divisions of a lateral 

 continuation of this transverse meristem contribute to the marginal 

 portions of the root cap. At a higher level, this lateral meristem 

 functions as a dermatogen and its ultimate divisions produce the 

 epidermis. The proximal face of the transverse meristem gives 

 rise to a massive plerome cylinder, and to a periblem which is 

 several layers in thickness. 



In the primary root, the stele is triarch (Fig. 175), rarely te- 

 trarch, and the protoxylem elements abut the pericycle, which occa- 

 sionally may be two or three cells in width at those points. The 



