STRUCTURE. 



cover two seeds with ease. Its cells (Fig. 41, c-e) are tubular and 

 stretched out in the direction of its growth, thin-walled, and somewhat 

 pointed. In N.flava they are about three times longer and wider than in 

 N. odorata and caerulea. The inner layer of cells adheres closely to the 

 seed, and its cells are much smaller than those of the outer layer. The 

 walls are supplied with fine pores (Weberbauer). The aril serves, by its 

 mucilage and inclosed air, to float the seed for a short time ; it drops the 

 seed in the course of a few hours (N. tuberosa) or a day or so (N. caerulea), 

 partly by being bursted and torn through absorption of water, partly by 

 decay (Fig. 41, b). 



The seed proper is protected by a firm shell consisting of two parts, 

 the outer a single layer of thick-walled, close-fitting cells, the inner a 



D 



Fio. 41. The aril. If. odorata.- a, diagrammatic section of aril and seed ; Z>, portion of aril 

 after the seed is dropped ; c, cell of the aril where the outer wall bends in, in continuity with 

 the inner wall of the aril ; cJ, cell of inner wall ; e, cell of outer wall. 



few layers of thick-walled, but loosely placed cells. The latter layer is 

 of the same thickness as the former ; its cells are much flattened tangen- 

 tially and contain large tangential air-spaces ; the walls are lignified, with 

 the exception of a thin layer next to the lumen. The cells of the outer 

 layer are also strongly compressed tangentially (except in N.flava). On 

 account of the great induration, which is thickest on the outer wall, they 

 have but a small lumen (Fig. 42). The walls are penetrated by pore- 

 canals. They are composed of lignin, excepting a thin layer of cellulose 

 next to the lumen and an outer thin cuticula which projects inward along 

 the boundaries of the cells. In the cells there are (in N. alba) bluish-green 

 granules which give color to the seed. In surface view the outer cells are 

 arranged in longitudinal rows, and, except at the micropylar end, they have 

 very sinuous walls. Over the raphe the cells are elongated in the direc- 

 tion of the rows, but elsewhere at right angles to the rows. At the micro- 

 pylar end of the seed the cells of the hard (outer) layer are nearly square 

 and in rows, but interrupted over the hilum, which lies (in N. alba] about 



