CHAPTER XXII. 



II. MUSCI. 



THE Mosses for the most part show greater uniformity of plan in their 

 sporogonia, and give less indication of the steps of their evolution than 

 do the Liverworts. It is a question open for discussion what, if any, 

 are the genetic relations of these two classes. Whatever view may be 

 held on this point, there are certainly strong features of similarity between 

 their sporogonia. Without necessarily accepting these resemblances as 

 indications of near genetic affinity, they must at least be held to point 

 a strong analogy between the two series : so strong indeed that it will 

 go far to justify an application of a theory of sterilisation in the Musci, 

 even where the gradual steps of the process are less clearly indicated 

 than they are in the Liverworts. 



The Musci include the Sphagnales, the Andreaeales, and the Bryales : 

 these are sufficiently distinct in their sporophyte-structure to require 

 separate description ; and the Sphagnales will be taken first, as showing 

 the clearest analogies with the Hepatics. 



A. SPHAGNALES. 



Notwithstanding the strong divergence of their gametophytes, both in 

 form and in structure, the sporogonia of the Sphagnales and Anthocerotales 

 show marked similarity, both in form and in development. Alone among 

 the Mosses the embryo of Sphagnum segments by successive transverse 

 walls, like a Liverwort : there is no continued apical growth, the further 

 enlargement after the first segmentations being intercalary in the segments 

 already laid down (Fig. 132 A): of these only the upper three or four 

 go to form the capsule : the rest form the short seta, and the foot. In 

 the upper region each segment divides into quarters, which again divide 

 so as to form in each transverse section a central group of four (endothecium), 

 and a peripheral series (amphithecium) (Fig. 132 c, n, E). The former 

 give rise to the columella alone, which is in Sphagnum a bulky mass 



