410 THE STRUCTURE OF ECONOMIC PLANTS 



/-ep hr 



— ep 

 —hr c 



— — CO c 



— lu 



Balls (4). Reeves (xy) has reached a different conclusion with 

 respect to this layer, on the basis of comparative studies of young 

 ovules; and states that "the fringe tissue originates as the inner 

 epidermis of the inner integument." The cells are polyhedral, 

 and the walls are irregularly thickened or fringed so that they 

 appear to be nodular or wavy. The second layer of the sheath 

 consists of thick-walled cells containing aleurone grains which 



are the remains of the endosperm. 

 The cotyledons are thrown into 

 complicated folds in their devel- 

 opment; but, when straightened 

 out, are broad and kidney-shaped. 

 p(,l I They contain very prominent resin 

 cavities and reserve foods, includ- 

 ing aleurone and fat from which 

 cotton seed oil is derived. 



Development of the Seedling. 

 — The cotton seedling develops 

 rapidly and the primary root pushes 

 its way through the micropyle of 

 the seed and grows without branch- 

 don: <?ir c, outer brown coat ; w c, layer ing for Several days. Measure- 



of colorless cells, r./, cotyledon; .«^, ^^^^^ . g^jj^ ^g>) indicated a 



endosperm ; ep, epidermis ; ep hi; epider- •' ^ -^ 



mal hair; g/, gland; ? ^r r, layers of inner grOWth in length of 6 Cm. in the 



brown coat; / .^ lower epidermis of coty- ''^^.^^ fQ^j. days,"and an additional 8 



ledon ; ///, lumen of palisade layer of seed . . , 



coat; />.// I, palisade cells of seed coat; cm. in the ensuing three days. 

 pa/ 1., palisade cells of cotyledon; psm. During this time, there is a prolific 



perisperm consisting of fringe cells ; ra, , , '' <- .... 



raphe; ., seed coat ; «./,, upper epider- development of root hairs in the 

 mis of cotyledon. (After Winton and region above the zone of elonga- 



tion. The straightening oi the 

 arched hypocotyl lifts the much 

 convoluted cotyledons out of the ground; and after being freed 

 from the seed coats, they expand rapidly. Coincident with this 

 development, lateral root formation occurs, and subsequent growth 

 of the epicotyl results in the elongation of the axis and the pro- 

 duction of the first foliage leaves. (Fig. 114.) 



The Primary Root. — The primary root has an exarch, radial 

 protostele which is generally tetrarch, although pentarch primary 

 roots may occur. (Figs. 115 A^ 116.) The protoxylem is com- 

 posed of spiral and annular elements that become much elongated 



pal 2 



u ep 



Fig. 113. Section of seed including seed 

 coat, perisperm, endosperm, and cotyle 



Winton, Structure and Composition oj Foods, 

 John Wiley and Sons, Inc.) 



