774 LIFE : OUTLINES OF GENERAL BIOLOGY 



exhausted by its exuberant reproductive expenditure, rapidly 

 withers away. 



Among the Algae and Fungi (the Thalloph3rtes) there are many 

 very striking life-histories, but it seems for our present purpose 

 more iastructive to begin at a higher level — ^with the Liver- 

 worts and Mosses (the Bryophytes). Here we find a well- 

 established alternation of generations of a type rather different from 

 that which sometimes occurs among Thallophytes and closely 

 akin to that which is found in the higher Ferns and Horsetails 

 (Pteridophytes). 



We have defined alternation of generations as the alternate 

 occurrence in one life-history of two different forms differently 

 produced. The one form produces spores, germ-cells requiring no 

 fertilisation, and is called a sporophyte; the other form produces 

 egg-cells and sperm-cells, and is called a gametophyte. In the cells 

 of the sporophyte the number of nuclear chromosomes is twice that 

 found in the cells of the gametophyte. In technical language the 

 sporophyte has the diploid (2w) number, and the gametophyte the 

 haploid (n) number. A meiotic or reducing division occurs in 

 the formation of the spores. The diploid number is restored in 

 the fertilisation of the egg-cell by the sperm-cell. 



LIVERWORTS. — On moist soil or rocks there are often extensive 

 growths of small flat plants or Hepaticse, which are interesting as 

 the probable representatives of the earliest terrestrial vegetation. 

 Air-pores on the upper surface of the prostrate thallus lead into 

 air-chambers into which there project numerous short filaments 

 with chlorophyll corpuscles — the seat of photosynthesis. In tjrpical 

 forms, like the common Marchantia, the under surface is colourless 

 and gives off absorbing root-hairs or rhizoids. The upper surface 

 often shows little cups in which minute buds or gemmae are 

 produced, and if these are washed away they develop into new 

 gametophytes. 



From the upper surface of the Marchantia thallus delicate stalks 

 rise, bearing a disc or receptacle on their summit. These are the 

 male and female sex organs, and the two kinds may occur on one 

 gametophyte as in Riccia or, as in Marchantia, on two separate 

 plants. The male receptacle bears flask-shaped cavities on its upper 

 surface, and in each of these there is a somewhat club-shaped male 

 organ or antheridium. Within a protective sheath there are sperm- 

 mother-cells which produce spermatozoa or antherozoids. Each of 

 these is a minute spirally twisted cell, ending in two motile cilia. 

 Moving in the water, which is usually abundant about the liverworts, 

 a spermatozoon finds an egg-cell and fertilises it. The egg-cell is 

 formed at the base of a flask-shaped archegonium, which consists of 

 a protective outer wall and internal germ-cells. The lowest of these 



