432 



PART IV, CLASSIFICATION. 



of a leathery consistency, produced annually or at a longer interval, 

 forming a crown at the top of the stem ; the foliage-leaves are generally 

 developed expanded, but in Cycas the pinnae are circinate in vernation, as 

 is also the phyllopodium in Stangeria and Zamia. 



The dioecious flowers are produced, either singly or several together, at 

 the apex of the stem ; they are cones (except Cycas). The development 

 of the cones does not arrest the growth in length of the stem : hence the 

 stem may be regarded as a sympodium, its growing-point being maintained 

 by either dichotomous or lateral branching (p. 18). The macrosporo- 

 phylls of Cycas do not constitute a true flower, since they are not borne, 

 as in the other genera, on a special axis, but simply take the place of a 

 whorl of foliage-leaves. The cones consist of an elongated axis, bearing 



numerous spirally- arranged 

 scaly sporophylls, which vary 

 in number from 30 to 600. The 

 microsporophylls bear on the 

 under surface usually numerous 

 (2 to 1000) microsporangia, either 

 scattered or in sori (Cycas, 

 Stangeria, Zamia). The macro- 

 sporophylls bear two orthotro- 

 pous macrosporangia, one on 

 each flank, developed upon the 

 peltate terminal lamina; but the 

 exceptional macrosporophylls of 

 Cycas (see Fig. 253) may bear as 

 many as 5-6 macrosporangia. 



The macrosporangia are all 

 sessile, and have a single in- 

 tegument, and are of consider- 

 able size ; those of Cycas are as 

 large as a plum before fertilisa- 

 tion. 



In the coniferous genera, the 

 macrosporangiate flower be- 

 comes the fruit; that is, a dry 

 cone, the sporophylls of which 

 fall away, and so set free the 



In Cycas, the sporophylls bend outwards and drop off, bearing the 

 seeds. The seed is covered by a testa, developed from the integument of 

 the ovule, which is succulent externally and stony internally. It con- 

 tains a single straight embryo, on a coiled suspensor, lying in the endo- 

 sperm. The embryo has generally two cotyledons (one in Ceratozamia, and 

 occasionally in other genera also), which are hypogean. 



The Cycadaceae, of which there are nine genera, and about seventy-five 

 species, are all tropical or subtropical. 



Order 2. Coniferae. This order includes the Pines, Firs, Cypresses, 

 Yews, etc., which, for the most part, are extra-tropical, inhabiting more 

 especially the northern hemisphere. 



Fi&. 253. Sporophylls of Cycads. A macro- 

 sporophyll of Cycas revoluta (J nat. size) : / 

 pinnae; s ovules. B Macrosporophyll of 

 Zamia muricata, with two ovules (s); C 

 microsporophyll of this species with numerous 

 inicrosporangia (p). 



