76 



ANCIENT PLANTS 



Fig- 55-— "Seed" 



In appearance this is lilie a seed, but differs from 

 a true seed in having no embryo, and is lilte the 

 preceding structure in having a very large spore, s, 

 though there is no trace of the three aborting ones. 

 The spore develops in a special mass of tissue known 

 as the nucellus, n, which partly corresponds to the 

 sporangium wall of the previous types. In it a, 

 cavity, pc, the pollen chamber, receives the pollen 

 grains which enter at the apex of the "seed" There 

 is a complex coat, c, which stands round the nucellus 

 but is not joined to it, leaving the space / between 

 them. Only in fossils; Trigonocarfus {see p. 122) 

 is similarly organized. Small spores in fern-like 

 sporangia, called pollen grains. 



Fig. 56.— "Seed" 



Very similarly organized to the above, but 

 the coat is joined to the nucellus about two- 

 thirds of its extent, and up to the level /. In 

 the pollen chamber, pc, a. cone of nucellai; 

 tissue projects, and the upper part of the coat 

 is fluted, but these complexities are not of 

 primary importance. The large spore s ger- 

 minated and was fertilized within the "seed",, 

 but apparently produced no embryo before it 

 ripened. Small "spores" in fern-like spo- 

 rangia form the pollen grains. Only in fossils, 

 e.g. Lagenostoma. (See p. 119.) 



Fig. 57.— Seed 



Essentially similar to the preceding, 

 except in the possession of an embryo e, 

 which is, however, small in comparison 

 with the endosperm which fills the spore 

 s. The whole organization is simpler than 

 in the fossil Lagenostoma, but the coat is 

 fused to the nucellus further up (see I). 

 Small "spores" form the pollen grains. 

 Living and fossil type, Cycads and 

 Ginkgo. 



