180LJ on British Mosses. 239 



phenomenon like that which occurs in reference to the Equisetacese 

 and Lycopodiaceie, viz. that the earliest fossil species known belong 

 to very highly-developed forms of the group. 



Life-History. — The following table (p. 240) is intended to illustrate 

 the life-history of a moss in its fullest and in its abbreviated courses, 

 and to bring this history into comparison with that of the ferns. 



Attention should first be drawn to the second column, which 

 shows the life history in its fullest form. It will be seen that it 

 starts with a spore and returns to a spore. 



From (1) the spore, which is a simple cell, proceeds (2) the 

 protonema, a line of cells, extending by transverse divisions, so that 

 it consists of single cells joined end to end to one another — an 

 organism indistinguishable from the hypha of an Alga. At points 

 this hypha throvrs off lateral branches which are always of less 

 diameter than the principal ones. There is thus produced a tangled 

 mat of fibres, running on or near the surface of the ground, and 

 often coloured by chlorophyll. It is the green stuff so often seen in 

 flower-pots which have been allowed to get too damp. At points in 

 the primary hypha individual cells begin to divide in a new fashion — 

 not by transverse septa as before, but by septa differently inclined, so 

 as to produce the rudiments of leaves ; and the direction of growth 

 changes from horizontal to vertical. Thus is formed (3) the hud, 

 which by growth gives rise to (4) the moss plant ; on this plant, some- 

 times in close proximity to one another, sometimes in different parts 

 of the same plant, sometimes on different plants, are formed (a) the 

 female cell or archegonium, and (b) the antheridia or male organs, 

 the antherozoids proceeding from which seek and find and fertilise 

 the archegonium. This completes the first part of the life of the 

 plant, the oophytic generation which results in a single sexual cell, 

 viz. the fertilised archegonium. From this cell arises the next 

 generation, consisting of the sporogone or stem bearing the capsule and 

 the capsule itself, in which without fertilisation are produced spores. 

 The plant has thus started with the spore, an asexual cell, reached 

 the point where its whole future is gathered up in a sexual cell, 

 which has produced an organism again producing an asexual cell : we 

 started with a spore, and have returned to a spore ; we have travelled 

 round a circle, divisible into two parts or generations, one sexual, the 

 other asoxual ; and we have therefore a case of alternation of genera- 

 tions. To make this statement more clear, it may be observed that a 

 generation is here spoken of as that part of the life of an organism 

 which intervenes between the two points at which its whole future is 

 gathered up into one cell ; that such a cell is sexual when it is the 

 result of the combination of two previously existing and independent 

 cells ; that such a cell is asexual when it is not the result of such 

 combination ; that an alternation of generation exists, whenever in 

 the complete cycle of existence or life-history there are two points at 

 which the whole organism is reduced to a single cell, and when the 

 forms of the organism in the two intervals of its development are 



B 2 



