424 FERTILIZATION AND FORMATION OF FP.UIT IN PHAXI'.P.oOAMS. 



The integuments of the ovule are altered into the seed-coat, which often exhibits 

 a differentiation into two layers. The inner layer takes the form of a delicate, 

 colourless membrane which is rarely hardened, or of a mucilaginous, readily-swelling 

 layer. The outer seed-coat or testa shows, however, a great variety of form. It 

 usually consists of several layers of cells, the successive layers of which are frequently 

 very differently fashioned. Sometimes they are soft and membraneous, sometimes 

 rigid and parchment-like, woody or horny, whilst in other cases they may be succu- 

 lent and fleshy, or converted into a mucilaginous, sticky envelope. The outmost 

 layer of the testa is usually brown, gray, and black, more rarely yellow, white, or red 

 in colour. The significance of the various slimy layers, which on moistening become 

 sticky, of the little pits and furrows, warts, spines, and the like in promoting the 

 firm anchoring of the seed on its germinating bed has already been fully pointed 



) 

 Fig. 319.— Salix polaris with opened fruits showing masses of hairy seeds escaping. 



out (cf. vol. i. pp. 614-620). Many seeds, in order that they may be distributed by 

 the wind, develop from the outmost layer of the testa wing-like appendages, as, 

 for instance, in the seeds of the Caryophyllaceous Lepigonum marginatum (see 

 fig. 318 5 ), in those of the Cinchona-tree (Cinchona, figs. 318 7 and 318 s ), in the 

 tropical Vochysia (fig. 318 6 ) and Cedrela (figs. 318 9 and 318 10 ), and many others. 

 This just-mentioned Vochysia-seed is also characterized by the curious wrapping 

 of the cotyledons upon one another (fig. 318 6 ). Often, again, for the same purpose, 

 the superficial cells of the testa grow out, forming a plume or plexus of silky or 

 cottony hairs, as in the Indian species of Cotton-plant (Gossypium herbaceum, 

 fig. 318 -), and in the cotton-producing Eriodendron (fig. 318 ] ). In the seeds 

 of the Oleander (Nerium Oleander) the hairs at the apex are longer than those 

 at the base, whilst in the Willow-herb (Epilobium) a delicate tuft of long silky 

 hairs is developed at the apex only. 



In a considerable number of plants there is developed from the base of the seed, 

 or from its funicle, a curious and special structure, which by the time the seed is ripe 

 envelops the seed like a mantle. This structure is known as the Aril. These arils 



