SPERMATOPHYTES 



117 



development comes to a standstill and the sporangium with the 

 enclosed embryo is set free. This structure is the seed. 'I'he 

 seed-bearing plants are called Spermatophytes and form a 

 branch coordinate with Thallophytes, Bryophytes and Pteri- 

 dophytes. In the Spermatophytes alternation of generation 

 occurs as in the Archegoniates but the gametophyte is re- 

 duced even further than in the higher Pteridophytes. 



293. Class I. Gymnospermae. — In the Gymnosperms the 

 sporangia are borne on the surface of modified leaves (sporo- 

 phylls). The microsporophylls are arranged spirally on a 

 short branch. The megasporophylls are usually similarly 

 arranged. The microspore (pollen grain) develops a rudimen- 

 tary prothallus of from one to three cells and the sperm cells 

 reach the megaspore through a tube developed by the game- 

 tophyte. The megaspore (embryo sac) develops a prothallus 

 of many cells and several archegonia. The latter consists of 

 a large egg cell and a few small neck cells. 



294. Order i. — The Cycadince are tropical palm-like plants, with an 

 unbranched trunk and a rosette of pinnate leaves. The sperm cells bear a 

 spiral band of cilia. The embryo consists of a suspensor, two cotyledons, 

 a plumule and a hypocotyl. 



295. Order 2. — The Ginkgoincc contain only the Japanese genus Ginkgo, 

 a deciduous tree with small fan-shaped palmately veined leaves. The 

 sperm cells are ciliated. The embryo forms no suspensor. 



296. Order 3. — The ConifercB comprise the "evergreens" and a few 

 deciduous trees. The pines, cedars, spruces, hemlocks, cypresses and 

 junipers are familiar examples. In the structure of the stem they ditTer 

 from most Spermatophytes in the absence of tracheai. The trachcidsarc 

 highly developed and take the place of trachea?. The sperm cells are not 

 ciliated. The embryo forms a suspensor. 



297. Order 4. — The Gnetince contain only three genera of exotic plants. 

 They constitute in many respects a connecting link between the CJynino- 

 sperms and Angiosperms. 



298. Class 2. Angiospermae. — The Angiosperms include 

 all the true flowering plants. The most distinctive character 



