i86 THE STRUCTURE OF ECONOMIC PLANTS 



is known as the "knee." One limb extends downward to the coty- 

 ledonary plate and the other to the terminal portion of the cotyle- 

 don which remains embedded in the endosperm. 



Continued elongation of the two limbs brings the sharp "knee" 

 above the soil surface; and then the growth of the two limbs 

 becomes unequal, the one that joins the cotyledonary plate growing 

 more rapidly than the one leading to the haustorial tip of the coty- 

 ledon. As a result of this unequal growth, the cotyledon assumes 

 the shape of a bow in which the descending limb is curved and the 

 ascending one is stretched like a bow string. The effect of this 

 tension is to draw the tip of the cotyledon out of the seed and above 

 the soil, so that it straightens, except for a slight kink which re- 

 mains at the original locus of the knee. In some cases, when the 

 soil is not too firmly packed, the seed may be lifted above the soil 

 by the tip of the cotyledon. By this time, the cotyledon is green 

 and photosynthetic. (Fig. 85.) The cotyledon attains a length 

 of about IX mm. ; and, at the end of a week, the primary root may 

 have grown downward from 1.5 to 5 cm. By the time seedling 

 development is complete, the root may reach a depth of about 

 10 cm.; but it does not branch, and all later roots are developed 

 adventitiously from the stem. 



Root hairs are produced in abundance; and, just above the 

 piliferous region at the top of the hypocotyl, the first two or three 

 adventitious roots emerge through the cortex. According to 

 Hoffman (7), they originate from the meristematic parenchyma of 

 the inner cortical region. The first of these roots arises in this 

 cortical sector just below the cotyledonary slit, the mode of emer- 

 gence appearing to be both lysigenous and schizogenous. The 

 second one usually originates at about a 90° angle to the right or 

 left of the first; and if a third one develops, it is in a position dia- 

 metrically opposite to the second. Subsequent adventitious roots 

 originate like the earlier ones, but there is no regularity with 

 respect to the order or place of their appearance. 



While the root and cotyledonary development is taking place, 

 the epicotyl is growing and the first foliage leaves elongate rapidly, 

 keeping pace with the enlargement of the basal sheath of the 

 cotyledon which surrounds them. Finally, the first foliage leaf 

 pushes its way through the small longitudinal slit in the cotyledon. 

 (Fig. 85, B.) By this time, the primordium of the second leaf has 

 developed, and emerges through a slit on the outer side of the 



