A Manual of the Mosses of Western Pennsylvania 

 and Adjacent Regions 



Introduction 



In a work containing keys and descriptions, so arranged as to make easier 

 the identification of the mosses of any region, it is desirable that a brief sketch 

 of the general life history of the mosses be included. In such a sketch it is 

 not necessary to enter upon a discussion of the many details of minute struc- 

 ture and behavior which, although interesting and important in themselves 

 and also for the light thus thrown upon genetic relationships, are yet of but 

 little practical value in a systematic manual where an easy and quick deter- 

 mination of the identity of the plant is the primary aim. 



Speaking broadly, the life history of m.ost of our mosses begins with a 

 minute single-celled spore, usually spherical in shape, which, under suitable 

 conditions, germinates and grows out as a slender thread or filament, which 

 upon further grovv'th may form a matted felt like layer, or may flatten out 

 into a more or less lobed body spoken of as a thallus, or may simply form a 

 solid cell mass, sometimes consisting of but a few cells. In either case the 

 structure resulting from the growth of the germinated spore is termed the 

 protonema. The protonema usually gives rise to buds, which in most mosses 

 grow to be the green leafy shoots which are ordinarily known as moss plants, 

 after which the protonema usually disappears. In a few of the mosses the 

 protonema persists indefinitely as a green felt-like layer on the soil or other 

 substratum. The stems of the green shoots resulting from the growth of 

 protonemal buds usually send out hair-like rhizoids which function as roots in 

 holding ihe plants in place and sometimes act as absorbing organs. The 

 leaver on these green shoots are sessile and with the exception of the midribs 

 (costae) are almost uniformly of but one cell in thickness. 



This whole phase in the life-history of a moss, beginning with the spore 

 and. including the protonema and the leafy shoot, is spoken of as the gameto- 

 phyte or se.\ual generation. The gametophyte is a sexual plant in that it 

 bears, in definite clusters surrounded by modified leaves called per^chaetial 

 leaves, the reproductive male and female organs which give rise respectively 

 to the sperm and egg. These clusters of reproductive organs surrounded by 

 more or less modified perichstial leaves are known as pertchaetia. When the 

 sperms and eggs are borne either in the same perich^tium or in different 

 perichaitia on the same plant the plant is spoken of as monoicoiis, but when they 

 are produced upon different plants, dioicous. When only male organs are in the 

 cluster the surrounding modified leaves are known as peri gonial leaves. 



The sperms are borne in a globose or more or less club-shaped sac, usually 

 mounted upon a stalk, and this sac is termed the antheridmm. When ripe the 



