OF WESTERN PENNSYLVANIA. 3 



^ins of ponds and small lakes. The general color is grayish 

 green, the stems are usually erect in dense tufts or mats and 

 bear at intervals fascicles of short and slender branchlets. The 

 capsules are usually more or less chestnut colored and globose, 

 while the leaves possess a peculiar and characteristic structure 

 consisting" of a meshwork of slender green cells enclosing in- 

 flated hyaline cells whose walls are more or less porose. 



The Andreaeales contain the one genus Andreaea, all be- 

 ing small tufted mosses growing on siliceous rocks in moun- 

 tainous regions. The capsule splits open by four vertical slits 

 which, however, do not reach the apex. 



The Bryales comprise by far the greatest number of the 

 mosses. Tlie capsule in the Bryales varies from globose to 

 ovate or pyriform or elongated cylindric. The cells which give 

 rise to the spores are known collectively as sporogenous tissue 

 and this tissue occupies but a small portion of the volume of 

 the capsule, being arranged in the form of a hollow tube or 

 cylinder vertically placed and open at both ends. The sterile 

 tissues occupying the hollow part of this tube constitute the 

 columella. The outer wall of the capsule usually contains 

 more or less green chlorophyll and the middle portion of this 

 wall is more or less loosely arranged and contains hollow 

 spaces. The capsule is covered by an epidermis, perforated by 

 sfomata in most mosses. The stomata are usually most highly 

 developed on the rounded or tapering base of the capsule 

 which is often more or less distinct and is known as the collum 

 or neck. In the ripening of the capsule the sterile tissues of 

 the wall and of the columella largely disappear, leaving the 

 capsule filled with a mass of spores. In some species the thin 

 wall of the capsule bursts irregularly, this type of dehiscence 

 being known as cleistocarpous. In other species the top of the 

 capsule separates as a lid or operculum. The separation of the 

 lid is often facilitated by the modification of a series of epi- 

 dermal calls termed the annulus, which usually becomes highly 

 hygroscopic and is often deciduous. The sterile tissues imme- 

 diately beneath the lid are usually more or less highly modified 

 to form a single or double series of pointed structures known 

 collectively as the peristome. The pointed structures con- 

 stituting the outer series in the double peristome or the single 

 series in a simple peristome are known as teeth, while the inner 

 and more delicate series of the double peristome are known as 

 segments. Between the individual segments are in many 

 species of mosses very delicate hair-like structures known as 

 cilia. Somtimes the cilia are in groups of two or more alternat- 

 ing with the segments. The peristome is usually very hygro- 

 scopic, curling" inward and closing the mouth of the capsule in 

 damp air and opening outward and allowing the free dispersal 



