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 determination of the identity 

 of the plant is the primary aim. 



Speaking broadly, the life history of a moss may be said 

 to begin 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 

 growth may form a matted felt-like layer, or may flatten out 

 into a more or less lobed body spoken of as a thallns, 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 proto- 

 nemal buds usually send out hair-like rhizoids which function 

 as roots in holding the plants in place and sometimes act as 

 absorbing organs. The leaves on these green shoots are sessile 

 and with the exception of the midribs (costae) are almost uni- 

 formly 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 gametophyte or sexual generation. 

 The gametophyte is a sexual plant in that it bears, in definite 

 clusters surrounded by modified leaves called perichaetial leaves, 

 the reproductive male and female organs which give rise re- 

 spectively to the sperm and egg. These clusters of reproduc- 

 tive organs surroundd by more or less modified perichaetial 

 leaves are known as perichaetia. When the sperms and eggs 

 are borne either in the same perichaetium or in different peri- 

 chaetia on the same plant the plant is spoken of as monoicous, 

 but when they are produced upon different plants, dio icons. 



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 antheridium. When ripe the antheridia absorb \vater and 



