Chapter 10 



THE SEED 



The seed, like many other plant structures, cannot be rigidly defined. 

 The term seed is loosely used, even in technical discussions; there is 

 little agreement even on such important characters as the presence of 

 an embryo and of dormancy in the embryo. The definition, "a seed is a 

 fertilized ovule," is inadequate, because it is applied to fertilized ovules, 

 both immature and mature. The presence of an embryo in some stage 

 of development is usually implied, but many fossil seedlike structures 

 that show no evidence of an embryo are termed seeds. And, in some 

 living gymnosperms — Ginkgo and the cycads — there may be no embryo 

 present when the seed is ripe and shed. (The ripe, plumlike "fruits" of 

 Ginkgo, which fall to the ground with the leaves, may be either ovules 

 or seeds, if the presence of an embryo determines a seed. Fertilization 

 frequently occurs on the ground.) But, in angiosperms, probably all 

 so-called seeds possess an embryo in some stage of development. Some 

 definitions of the seed include dormancy of the embryo as an essential 

 character. But, in many taxa, the embryo continues to grow within the 

 seed following its shedding, without a dormant period; a seed may be 

 "dormant" as far as external structure goes, but growing internallv. 

 Some angiosperm seeds apparently have no period of dormancy, sug- 

 gesting a survival of a primitive stage in seed evolution — some tropical 

 genera (Myristica, Diirio); some aquatic genera (Thalassia). Dormancy 

 of the embryo seems to represent an advanced stage in the evolution of 

 the seed. 



Seeds are commonly called "ripe" when they are fully grown and 

 shed from the mother plant, regardless of the stage of development of 

 their embryos; seeds have been called technically "ripe" only when 

 ready to germinate. After-ripening — morphological and physiological — 

 may delay readiness to germinate long after shedding. The term seed 

 must remain in morphological use and will commonly be applied to 

 matured ovules that contain an embryo at some stage. A loose use of 

 the term for seedlike fossils that may be either ovules or seeds is neces- 

 sary, as it is for the ripe "fruits" of Ginkgo. 



Size of Seeds 



The size of seeds has been considered as perhaps of phylogenetic im- 

 portance. Both large and small seeds have been called primitive. Large 



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