646 LECTURE XXIII. 



absolutely necessary in such a case. In some plants it suf- 

 fices that the nuclear substance should be derived from two 

 organs borne by the same individual, in which case the sexual 

 process is one of self -fertilisation. In some plants, such as the 

 Fungi which have pollinodial antheridia (Peronosporeae and 

 .some Ascomycetes), self-fertilisation alone is possible. But 

 in most cases the conditions under which the sexual process 

 is effected, for instance, the formation of free-swimming piano- 

 gametes and antherozoids, and of spermatia and pollen-grains 

 which are readily transportable, are such as to render possible 

 the fusion of sexual cells derived from two distinct indi- 

 viduals, that is, cross-fertilisation. In some cases there are 

 special arrangements for ensuring cross-fertilisation, the most 

 general of which is dicecism ; that is, the production of the 

 male and female organs by distinct individuals. Thus in 

 certain Fucaceae (Fucus vesiculosus, nodosus, serratus, Himan- 

 thalia lored] some individuals have only antheridia and others 

 only oogonia; in Spirogyra and other Zygnemeae the cells of 

 one filament act as male organs, those of the other as female 

 organs ; among the Muscineae the plants frequently bear only 

 either antheridia or archegonia ; in the Isosporous Vascular 

 Cryptogams the prothallia are usually hermaphrodite, but 

 exclusively male or female prothallia occur not infrequently 

 in the Filices, and as a rule in the Equisetaceae. In hetero- 

 sporous plants diceeism is brought about in a somewhat dif- 

 ferent manner. These plants, as mentioned in the previous 

 lecture (p. 603), have two kinds of spores, macrospores and 

 microspores ; the former always give rise on germination 

 to a female (archegoniate) prothallium, the latter to a male 

 (antheridial) prothallium ; hence the male and female organs 

 are necessarily borne by distinct individuals. 



It is usual to speak of these Phanerogams only as being 

 dioecious in which the microspores (pollen-grains) and ma- 

 crospores (embryo-sacs) are borne by distinct individuals; 

 but this usage requires explanation. All Phanerogams, being 

 heterosporous, are essentially dioecious, for the sexual process 

 takes place between the two distinct individuals represented 

 by the pollen-tube on the one hand, and by the more or less 



