i86 LIFE : OUTLINES OF GENERAL BIOLOGY 



tvpc. such as a fern, the strong vegetative plant (the sporophyte) 

 produces spjres which are all alike (isogamous). They fall to the 

 ground and in favourable conditions each may develop into a small 

 and delicate prothallus (gametophyte), which bears sexual reproduc- 

 tive organs (archegonium and antheridium, or ovary and spermary). 

 An egij-cell in the archegonium is fertilised by a sperm-cell from the 

 antht-ridium. and develops into a young sporophyte. This is the 

 typical alternation of generations, which is discussed elsewhere. 



{d) It is elsewhere explained that in the seed-bearing plants, from 

 the old-fashioned Cycads to the Conifers, and from these to the 

 Phanerogams with their true flowers, there is a persistent, though 

 much masked, alternation of generations. But from the ecological 

 jx)int of view it is desirable to keep the seed-bearing plants in a 

 group by themselves. For this there arc two reasons, first, because 

 of the practical suppression of the gametophyte generation as a 

 separate phase; and second, because what is separated from the 

 maternal plant, or from the female portion of the parental plant, 

 is a seed, which is already a young plant (an arrested sporophyte) 

 that has Ix^en living for some time in dejx'ndence on the parent, and 

 is lilx^rated in a manner comparable ecologically to the viviparity 

 of mammals. F'rom the standpoint of comparative embryology, the 

 flowering plant shows alternation of generations with a much 

 reduced gametophyte stage; but the development, equipment, and 

 disjx'rsal of the seed is so unique that the separation of seed-plants 

 from all others is warranted as an ecological convenience. 



ECOLOGY OF THE FLOWER. -Morphologically, as Goethe 

 and others recognised at the end of the eighteenth century, the flower 

 is a shortened region of the plant-axis, bearing a number of trans- 

 formed foliar organs, some of which (stamens and carpels) are 

 sjx'cialised as s|X)re- producers. Typically there are four whorls, tiers, 

 or series of floral structures, all of which are homologous with 

 leaves: the sepals (forming the calyx), the petals (forming the 

 roroUa). the stamens (forming the andnrcium), and the carpels 

 (forming the gyn^rcium or pistil). 



Phvsiologically regarded, the flower is the reproductive part of 

 a seed-plant, the stamens prorlucing microspores, which give rise to 

 pollen grains, and the carjx'ls producing macrospores (or mega- 

 spores), each known as an embryo-sac. A male nucleus arises within 

 the |x>llcn-grain or within its outgrowth the pollen-tube, which 

 actually represents the much-reduced male gametophyte. A female 

 nucleus arises within the embryo-sac, which actually represents the 

 much-reduced female gametophyte. The union of the two gametes 

 forms a zygote, which proceeds to develop into a young sporophyte, 

 the embryo plant within the ovule. It is no longer regarded as 

 stfictly accurate to speak of the stamens as male organs and of the 



