THE DEVELOPMENT OF ORGANISMS 777 



strikingly different. The aberrant Isoetes or quillwort is aquatic or 

 amphibious, with stunted stems and long quill-like leaves. We shall 

 begin with the life-history of a typical Lycopodium. 



The simplest Lycopodium sporophyte is a leafy stem, with every 

 leaf a sporophyll, that is to say, bearing a sporangium on its upper 

 surface. But in other species there is a division of labour between the 

 lower foliage leaves and the upper sporophylls, which are closely 

 clustered into a "strobila". In the centre of the stem in the young 

 stages and simpler types there is a cylinder or stele, consisting of a 

 core of wood or xylem (water-conducting tissue) and a sheath of bast 

 or phloem (food-conducting tissue). This is the simplest form of 

 vascular system. The central cylinder gives origin to branches or 

 leaf-traces which pass out into the veins of the leaves. 



A spore falling from a sporangium gives rise to the gametophyte, 

 which typically shows a complex subterranean tuberous body (the 

 primary tubercle), and an above-ground green portion (the crown), 

 which bears the sex organs. The tuberous part always contains a 

 partner fungus (see Mycorhiza) ; but this region is suppressed in some 

 epiphytic Lycopodiums, whose gametophyte is therefore altogether 

 aerial, though not always green! In other species, the crown is 

 rudimentary or absent, and the sex organs arise directly from the 

 tuberous body. These are interesting alternatives, showing a sup- 

 pression of the rooting region in one case and of the photosynthetic 

 green shoot in the other. 



The sex organs consist of antheridia and archegonia imbedded in 

 the upper surface of the crown. The sperm-cells are unusual among 

 Pteridophytes in being biciliate as in Bryophytes. After fertilisation 

 the egg-cell divides into a suspensor cell (very characteristic of seed 

 plants) and an embryo-cell. The latter divides into four unequal 

 quadrants, two of which form a foot (as among Bryophytes) 

 which absorbs food for a time from the gametophyte, while the 

 two others give rise to the stem and the first leaf (plus root) 

 respectively. 



Among the liverworts, or, as some would say, beside the liver- 

 worts, there is a peculiar group, whose type is Anthoceros. The most 

 outstanding peculiarities are that the sporophyte is beginning to be 

 green, which spells independence; that the spore-making tissue is 

 beginning to be divided into compartments, anticipatory of spor- 

 angia; and that there is a hint of division of labour between spore- 

 making and non-reproductive tissue. Bower has pointed out that if 

 the green tissue increased in proportion, and gave off minute exten- 

 sions, each with a spore-making patch at its base, there might arise 

 something like the strobila of a Lycopod ; and we have already 

 mentioned that there are among species of Lycopodium various 

 gradations from a sporophyte with sporophylls only and one with a 

 considerable development of foliage leaves as well. 



