368 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



The variety of fruits is almost as great as that of flowers, and must 

 be considered an important factor in the success of the angiosperms, 

 as well as one of their prime benefits to humanity. It seems more 

 than a coincidence that the evolution of a group of plants of the ca- 

 pacity of the angiosperms, in which, as in some of the cereals, 30 per 

 cent of the total weight of the plant is stored as elaborated food in the 

 seeds, should have been contemporaneous with the evolution of the 

 warm-blooded animals. At any rate, it seems certain that human 

 civilization could not have evolved but for the evolution of this plant 

 phylum. 



The angiosperms are so numerous and present so many morpho- 

 logical diversities that it is impossible to give a succinct characteriza- 

 tion. Their outstanding feature is angiospermy itself, i. e., pollina- 

 tion results in bringing the pollen spores in contact with a receptive 

 surface of the carpel (stigma), and not with the ovule, as in all 

 other known seed plants. All known angiosperms have closed 

 ovaries, and no other phyla have. At the same time it should be 

 recalled that angiospermy was itself a product of evolution and that 

 some time a series of fossil forms may be discovered showing this 

 character in the formative stage. 



Anatomically angiosperms are characterized, the dicotyledons nor- 

 mally and the monocotyledons primitively, by the vascular system 

 of the stem constituting a tubular cylinder (siphonostele) of col- 

 lateral bundles and having leaf gaps. The wood is marked by the 

 presence of vessels arising through cell fusion — a feature common to 

 the gnetalean coniferophytes and one absent in certain specialized 

 families of angiosperms such as the Cactaceae and Crassulaceae, and 

 in other possibly primitive families such as the Trochodendraceae. 

 The improved conductive and supporting tissues of the angiosperms 

 result in a general improvement in storage tissues and in a great 

 expanse of foliage, and consequently a greatly increased production 

 of nutritive materials. These histological features as well as the 

 floral morphology, to be alluded to presently, are features of the 

 sporophyte generation. The history of the male gametophyte dot^ 

 not differ materially from that which obtains in the various gymno- 

 sperm phyla; but the female gametophyte is not only more reduced, 

 but its development is associated with new and peculiar features. 

 Free nuclear division in the egg is wanting (a feature common to 

 Gnetum and Tumboa among the Gnetales). The process of what is 

 commonly called double fertilization is a strictly angiospermous 

 characteristic. Among the units organized in the embryo sac is 

 what is known as an endosperm nucleus. One sperm unites with 

 the egg nucleus, and the result of this union is the embryo. The 

 second sperm nucleus unites with the endosperm nucleus, and the 

 result of this union is the trophophyte or endosperm which furnishes 



