938 PRINCIPLES OF STRATIGRAPHY 



but a thallus is formed closely applied to the substratum. The at- 

 tachment of the bryophytes is by rhizoids, true roots being ab- 

 sent. These rhizoids resemble the root-hairs of higher plants. 

 The reproductive organs are antheridia and archegonia, serving 

 for sexual reproduction. The former are stalked and develop the 

 spermatozoids, while the archegonia are flask-shaped, with long 

 neck, the egg-cell lying at the bottom. From the fertilized ovum a 

 capsule arises, generally borne on a stalk, and within this the 

 spores are developed. There is thus an alternation of generation 

 — the sexual stage, or gauictophytc, developing from the spore, and 

 the asexual, or spore stage {sporogoniuui or sporophyte), develop- 

 ing from the fertilized tgg of the gametophyte, and, in turn, pro- 

 ducing the spores. The spore-bearing generation is throughout life 

 dependent on the gametophyte, whereas in pteridophytes it becomes 

 an independent plant. The order Sphagnalcs contains the single 

 genus, Sphagnum, with numerous species known as bog-mosses. 

 The order Andrccvales also contains a single genus, Andresea, for 

 the most part an Alpine and Arctic plant, growing on bare rocks. 

 The order Phascales includes a few small species, chiefly of the 

 genus P.hascum. The order Bryalcs, on the other hand, contains a 

 very large number of genera and species. 



Fossil mosses, especially of the genus Hypnum, have been ob- 

 tained from the Miocenic and Quaternary deposits of Europe and 

 the Arctic region, and also from western America (Green River 

 beds). They are doubtfully represented in Mesozoic and earlier 

 deposits. 



Phylum W — Pteridophyta. The pteridophytes, or vascular 

 cryptogams, form the highest division of the flowerless plants. 

 Their internal vascular structure allies them with the higher plants. 

 In them alternation of generation has been carried farthest, in 

 that the first stage to develop from the germinating spore is the 

 gametophyte, known as the protliallns. This is a small, flat, green 

 plant-organism which carries on its under side the archegonia and 

 autiieridia, together with the rootlets or rhizoids. This sexual 

 plant is independent of the sporophyte or asexual generation, 

 while the latter at first draws nourishment from the prothallus but 

 becomes physiologically independent when its roots develop. This 

 independence of the two generations is the distinctive feature of 

 the pteridophytes, whereas in bryophytes the sporophyte is through- 

 out its life attached to the gametophyte, while in the spermaphytes 

 the more or less reduced gametophyte remains enclosed within the 

 tissues of the sporophyte. 



The Equisctalcs, including the single living genus. Equisct'Mu, 



