CHAPTER XL — THE BRVOPHYTA. 



339 



The spores are minute cells with the wall differentiated into 

 two parts, a thicker outer coat, called the exospore, and a thinner 

 interior one, which is very distensible, like the inner coat of a 

 pollen-grain, called the endospore. In germinating, the exospore 

 ruptures, and the endospore becomes distended into a tube, 

 which usually divides transversely and forms a mass of green, 

 branching filaments, which resemble some of the filamentous 



Fig. 548. — Germinating spores and protonema of Moss, a, a, spores in different stages 

 of germination; b, filaments of the protonema: c, bud on protonema destined to develop 

 into a leafy plant. Magnified about 550 diameters. 



algae ; this is the protonema, from which either directly or by 

 production of lateral buds, the plant of the sexual generation is 

 developed. See Fig. 548. 



The Series is subdivided into two Classes, the Hepaticce and 

 the Musci. 



Class I. — The Hepatic* (Liverworts). 



The Hepaticae, or Liverworts, are the lower in organization. 

 They include many thalloid forms, and the leaf-bearing or foliose 

 ones present a simpler structure than the Mosses, proper. Their 

 capsules, if they dehisce at all, open lengthwise, usually into four, 

 but sometimes into two, valves. Except in one order, the Antho- 

 ceroteae, they do not possess a columella. Most of the species 



