370 MORPHOLOGY OF THE ANGIOSPERMS 



size seems often correlated with the tree habit, especially with the 

 trees of tropical forests. But large seeds often have the most highly 

 developed embryos and are frequent in temperate-climate forests. Small 

 seeds have been interpreted as most primitive because their embryos 

 are usually little developed. But, in many seeds, after-ripening brings 

 about greater development before germination. Seeds of medium size 

 with well-developed embryos have also been called primitive. Size of 

 seed seems less important, from the standpoint of primitiveness, than 

 time of inception of dormancy and stage of specialization of the embryo. 

 Since there is no period of dormancy in the seeds of some gymnosperms 

 — cycads and Ginkgo — and of a few angiosperms, and since there is a 

 long-continuing, after-ripening development in some primitive families, 

 absence of dormancy seems probably primitive. (After-ripening may 

 merge with germination.) Small seeds and after-ripening are associated 

 chiefly with herbaceous plants and with a short period of seed matura- 

 tion. 



Structure of the Seed 



The basic structure of the seed is that of the ovule, but, as the ovule 

 matures, some parts may be lost and the nature of others obscured. In 

 seeds formed from anatropous and campylotropous ovules, some evi- 

 dence of ovule type is commonly present, but appendages and his- 

 tological modifications may bury evidence of ovule type. In endosperm- 

 less seeds, form may be determined by the presence of a large embryo, 

 as in many legumes. Position of the micropyle may be evident as a 

 minute opening or concavity, and the place of abscission from the 

 funicle as a scar, the hiluni. (In early descriptions of the ovule, the 

 term hilum was sometimes applied to the enlarged tip of the funicle. ) 



The Seed Coats. The integuments of the ovule form the seed coats or 

 testa of the seed. As the ovule enlarges, tissues of the integuments 

 undergo great changes. The inner integument is commonly reduced and 

 often lost; the outer may become massive and highly differentiated as a 

 protective structure. Histologically, the coats may consist of few or 

 many layers, which vary in different taxa in their relation to the ovular 

 integuments (Fig. 137). Of the ovular integuments, some layers de- 

 generate; others increase greatly in number and kinds of tissues. Mis- 

 interpretations of the integumentary make-up of seed coats have been 

 made through failure to note loss of one integument in ontogeny. An 

 inner integument that appears to be lost may be reduced to a single 

 layer of delicate cells. Both integuments persist in some families — 

 Rosaceae, Rutaceae, Euphorbiaceae — and remain more or less distinct 

 in the mature seed. In the bamboos, absorption of the integument be- 

 gins immediately after fertilization and is reduced to a single layer of 



