. Morphology of gymnosperms. Gymnosperms; Plant morphology. ii6 MORPHOLOGY OF GYMNOSPERMS. v^:^^^^^ The sporophylls are spirally arranged in a compact strobilus, and show only slight traces of the pinnate character of the vegetative leaves. At the apex and base of the cone the sporophylls are sterile, and the remaining sporophylls bear abaxial sporangia in two more or less distinct groups separated by a median sterile line (fig. 99). In the median region of the cone the sporangia cover the entire abaxial sur- face of the sporophylls and sometimes even their margins. In all the genera the spora
. Morphology of gymnosperms. Gymnosperms; Plant morphology. ii6 MORPHOLOGY OF GYMNOSPERMS. v^:^^^^^ The sporophylls are spirally arranged in a compact strobilus, and show only slight traces of the pinnate character of the vegetative leaves. At the apex and base of the cone the sporophylls are sterile, and the remaining sporophylls bear abaxial sporangia in two more or less distinct groups separated by a median sterile line (fig. 99). In the median region of the cone the sporangia cover the entire abaxial sur- face of the sporophylls and sometimes even their margins. In all the genera the sporangia occur in soral groups of five, four, or three; but some sori may contain only two sporangia, and in the more highly specialized forms some of the spo- rangia may occur singly. The sori are surrounded by one-celled or two-celled hairs, and such hairs appear even on the lower portion of the sporangia (figs. 100, in). The number of sporangia on a sporophyll corresponds roughly to the size of the sporophyll. The following estimates have been made by Miss F. Grace Smith (56) and by Cham- berlain (63): Dioon spinulosum 750, Cycas circinalis 700, Encephalartos Coffer 700, Macrozamia Miquelii 600, Encephalartos villosus 500, Dioon edule 300, Ceratozamia mexicana 250-300, Stangeria paradoxa 260, Microcycas calocoma about 120, Bowevm spectabilis 67. A large number of sporangia on a sporophyll, and also the larger number in a sorus, may be regarded as primitive features, while fewer sporangia on a 'sporophyll and fewer in a sorus are probably due to reduction. The first full account of the development of the microsporangium was published by Treub (11) in 1881, who investigated especially Zamia muricata. Lang's (22) work on Stangeria paradoxa confirmed Treub's results in a general way and cleared up some doubtful points. Later, Miss F. Grace Smith (56) studied abundant material of very Fig. 90.—Dioon edule: photo- graph of two microsporophylls; X^.—After Chamberlain (63).. Please
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