THE PRIMITIVE ANGIOSPERM EMBRYO 365 



The interpretation of the cotyledon of the most advanced embryo 

 types of the monocotyledons as consisting of scutellum plus the 

 coleoptile goes back at least to the middle of the nineteenth century; it 

 is so described in the textbooks of botany of that period. At the end of 

 the nineteenth century, the monocotyledon embryo was remarkably well 

 interpreted, but the early twentieth century saw the sound interpreta- 

 tion replaced by others now seen to be incorrect. 



Two types of primitive cot)'ledon seem to be present in the mono- 

 cotyledons: the cylindrical or ligulate type, without differentiation of 

 lamina and base — Helobiales, Pandanales, some Liliales; and the leaflike 

 type, with lamina and petiole — some Liliaceae (Paris, Trillium). In 

 each of these types, absorbing sti-uctures have arisen independently by 

 modification of the cotyledon tip. In the dicotyledons, the cotyledon is 

 more leaflike in form, and only rarely is a suctorial structm-e formed. 

 Absorption of food stored in the endosperm as a function of the cotyle- 

 don has doubtless arisen several or many times in angiosperms. 



THE PRIMITIVE ANGIOSPERM EMBRYO 



Both the minute, little-developed embryo and the polycotyledonary 

 embryo have been considered the primitive type — the undeveloped 

 embryo, because of its simplicity, and the polycotyledonary embryo, 

 because of resemblance to supposedly primitive conifers with several 

 cotyledons. Polycotyledony, syncot)'ledony, schizocotyledony, and 

 monocotyledony all appear to represent modifications of dicotyledony. 

 The minute, undifferentiated embryo, embedded in abundant endo- 

 sperm, seems to be primitive; the well-developed embryo, with large 

 cotyledons and no endosperm, specialized. The undeveloped embryo 

 that continues development (after-ripening) in the ripe seed — under 

 favorable conditions of water supply and temperature — resembles that 

 of some gymnosperms — Ginkgo, the cycads — where the embryo grows 

 continuously after seed shedding. 



The seedlings of a few angiosperms, in contrast with the usual rapid 

 development, remain entirely underground the first growing season after 

 germination — species of Vc rat rum and Arum. In this character, they 

 resemble the sporophytes of Botrijchium, Ophio^ossum, and Lyco- 

 podium. Slow development may be a primitive character or the result 

 of after-ripening outside the seed. 



BIBLIOGRAPHY 



Artschwager, E.: Development of flowers and seed in the sugar beet, Jour. Agr. 



Res., 34: 1-25, 1927. 

 , E. W. Brandes, and R. C. Starrett: Development of flower and seed of 



some varieties of sugar cane. Jour. Agr. Res., 39: 1-30, 1929. 



