THE SEED. 



facilitate their dispersion by the wind ; whence winged seeds are 

 only met with in dehiscent fruits. For the same purpose, the 

 testa is sometimes provided with a tuft of hairs at one end, 

 termed a Coma ; as in Epilobium, Asclepias, or Milkweed (Fig. 

 963), and Apocynum (Fig. 954). In the Cotton-plant, the 

 whole testa is covered with long wool. It should likewise be 

 noticed, that the integument of numerous small seeds (and also 

 seed-like achenia) is furnished with a coating of small hairs con- 

 taining spiral threads (one form of which is represented in Fig. 

 31), and usually appressed and confined to the surface by a film 

 of mucilage. When the seed is moistened, the mucilage softens, 

 and these hairs shoot forth in every direction. They are often 

 ruptured, and the extremely attenuated elastic threads they contain 

 uncoil, and are protruded in the greatest abundance to a very con- 

 siderable length. This minute mechanism subserves an obvious 

 purpose in fixing these small seeds to the moist soil upon which 

 they lodge, when dispersed by the wind. Under the microscope, 

 these threads may be observed on the seeds of most Polemonia- 

 ceous plants, and the achenia of Labiate and Composite plants, as, 

 for example, in many species of Senecio, or Groundsel. 



623. The inner integument of the seed, called the TEGBIEN or 



a ENDOPLEURA, although frequently very 

 obvious (as in Fig. 451), is often indis- 

 tinguishable from its being coherent with 

 the testa, or else altogether wanting. 

 Nor when present does it always origi- 

 nate from the secundine or inner coat of 

 the ovule (563). In the Hypericum Family (Fig. 454), in the 

 Pea Family, and probably in a great many other cases, especially 

 where it is tumid or fleshy, or where it adheres firmly to the albu- 

 men, it doubtless consists of the remains of the nucleus of the 

 ovule, or of the embryo sac. 



624. The stalk of the seed, as in the ovule from which it origi- 

 nated, is called the FUNICULUS (Fig. 452). The scar left on the 

 face of the seed by its separation from the funiculus at maturity is 

 termed the HILUM. The relation of the hilum to the chalaza, mi- 

 cropyle (563), and other parts of the seed, has been sufficiently 



FIG. 451. Vertical magnified section of the (anatropous) seed of the American Linden: a, 

 the hilum; b, the testa; c, the tegmen; d, the albumen; e, the embryo. 4-52. Vertical section 

 of the (orthotropous) seed of Helianthemum Canadense : a, the funiculus. 



