284 



OUTLINES OF PLANT LIFE. 



the seed chance to alight upon water, or they may suffice to 

 entangle the seed in the fur of animals. 



400. Adaptations for germination,— Adaptations for dis- 

 tribution not infrequently also secure advantage in germina- 

 tion. It is important for 

 many seeds that they be 

 anchored to the ground when 

 they have once been trans- 

 ported, so that they may not 

 be subject to further disturb- 

 ance. Such anchorage is 

 sometimes secured by the 

 transformation of the outer 

 layer of cells into mucilage, 

 so that the seed, upon be- 

 coming wet, is stuck fast to 

 the soil ; or by the tufts of 

 hair which, once wetted, 

 cling to the surface of the 

 earth ; or by barbed bristles 

 and hygroscopic awns which, 

 having become entangled 

 among the grass, work a 

 pointed seed body deeper 



by every change of moisture (fig. 241). 



401. Summary. — Plants have developed many ways for 

 protecting and distributing their spores and seeds. Pollen is 

 often protected against rain by closure of the flower-leaves or 

 bending of the stalk. Fungus spores may be shot off or 

 slung off. Many ferns sling out their spores from the cases. 

 Water and air currents carry spores. Insects are also efficient 

 distributors, especially for the seed-plants, which provide food, 

 shelter, nest-building materials, etc., to secure their aid. 

 This they advertise by color and odor. By irregular form 



Fig. 249. Fig. 250, 



F'lG. 249. — A, cluster of fruits of Spanish 

 needles {Bideits hipinnata). B, a single 

 fruit enlarged, showing barbed awns, reja- 

 resenting the calyx lobes, by which it 

 adheres to animals. A , natural size ; B, 

 magnified z^ diam. — After Kemer. 

 Fig. 250. — Fruit of cockle-bur {Xanthium 

 strumariu m), halved, showing two seeds, 

 the upper of which usually germinates a 

 year later than the lower. Natural size 

 — After Arthur. 



