ITS COATS AND APPENDAGES. 



307 



uated long threads (spirides) which were coiled within. These, 



protruding in all directions and in immense numbers, form a 



limbus of considerable size around the seed, and evidently must 



serve a useful end in fixing thsse aual 



and light seeds to the soil in time of 



rain, or to moist ground, favorable to 



germination. In cress and flax-seed, 



the abundant mucilage developed when 



wetted comes from the gelatination of 



epidermal cell-walls, and 



subserves a similar use. 



594. While the testa in 

 many seeds is hard and 

 crustaceous or bony, imitat- 

 ing the pericarp of a nut, in others (such as Paeonia) it becomes 

 berry-like (baccate), and in Magnolia, drupaceous. 1 (Fig. 668- 

 671.) These may also be regarded as adaptations for dissemi- 



nation, here by the agency of birds, attracted by bright coloring 

 and edible pulp. 



595. The rhaphe of an anatropous seed (shown in Fig. 681, 

 685) is sometimes so salient as to form a conspicuous appen- 

 dage, as in Sarracenia, Fig. 672. Again it may be wholty 



1 See article On the Structure of the Ovule and Seed-coats of Magnolia, 

 in Jour. Linn. Soc. ii. 106, from which the accompanying figures and Fig. 

 589-597 are reproduced. 



FIG. 668. Forming seecl (one eighth of an inch long) of Magnolia Umbrella; the 

 rhaphe toward the eye. 669. Magnified view of the same divided lengthwise through 

 the rliaphe; the outer mat, a, beginning to form a hard inner layer, nf. Within and 

 distinct from this is the inner coat (b\ immediately enclosing the nucleus, c. The oppo- 

 site side of the testa is thicker on account of the rhaphe, in which d indicates the cord 

 of spiral ducts. 



FIG. 670. A nearly full-grown seed, of the natural size 671. Longitudinal section, 

 emargeo, snowing the crustaceous or stony inner stratum of the testa well developed: 

 the parts lettered as in Fig. 669. 672. A transverse section in the same position. 



