GYMNOSPERM^. 



121 



sexual generation has gradually been suppressed as an independent existence. The spore 

 and the prothalliuni have become one, and the sexual stage reduced to a few cells, repre- 

 senting the prothallium, developed in the embryo-sac in the seed and in contact with the 

 rudiments of a female prothallium. The pollen grains of Gymnosperms, are according to 

 Sachs, homologous with the microspores of Selaginella, since they are subdivided internally 

 into cells representing a very rudimentary male prothallium. The largest of the cells 

 ruptures and becomes transformed into the pollen-tube. The pollen-sacs containing the 

 grains of pollen are produced on the inner sides of staminal leaves, either in pairs or in 

 larger groups, and open at maturity to set free their contents. The macrosporangia are each 

 reduced to a single macrospore called a nucellus or ovule, and may be formed either of 

 the metamorphosed end of the floral axis ; or be axillary ; or proceed from the carpels. The 

 carpels never cohere so as to form a true ovary before fertilisation, hence the term Gymno- 

 sperm ; although on ripening they often increase in size so considerably that they close 

 together and conceal the seeds. Cases are, however, " not rare in which the seeds remain 

 quite naked from first to last."^ One or more embryo-sacs are formed near the apex of 

 the ovule or seed. An endosperm or mass of cellular tissue corresponding to the prothal- 

 lium of Cryptogams is formed in the embryo-sac, and within this arise Archegonia or 

 Corpuscula. The pollen-tubes penetrate the ovule and reach the corpuscle, impregnating 

 the germinal cell within, called the oosphere, in which the embryo of the future plant 

 subsequently appears. " During the development of the embryo the endosperm (prothal- 

 lium) becomes filled with nutrient materials and increases greatly in size ; the embryo-sac 

 which encloses it grows at the same time, and finally entirely absorbs the surrounding 

 tissue of the nucellus (ovule) ; the integument, or an inner layer of it, becomes developed 

 into a hard shell, while frequently (in naked seeds) its outer mass of tissues becomes 

 fleshy and pulpy and gives the seed the appearance of a drupaceous fruit {e.g. 

 Cycas, Salisburia). The eff'ect of fertilisation extends to the fertile leaves, which enlarge, 

 swell, harden, and approach each other, and constitute a complex organ, the fruit forming 

 fleshy or woody coatings to the seeds {e.g. Juniper), or woody supports beneath them as 

 in Conifers."" The reproductive organs of the Gymnosperms are thus seen to be very 

 complex. They are simplified in the still higher Angiosperms by the final obliteration of the 

 Archegonia or Corpuscula and of the endosperm or prothallium contained in the embryo- 

 sac, whilst the pollen grains or microspores become almost unicellular, and thereby lose 

 the last trace of the inclosed male prothallium of the Lycopodiaceae. The seed develops 

 gradually as it ripens, its function being to protect the embryo and to store up nutriment 

 for its use when germinating. The embryo lies in the endosperm which fills the seed and 

 is differentiated distinctly into stem, root, and leaves. 



The "true Gymnospermse," comprehending the order Conifer^e only, have a branching 

 stem, usually arranged in verticels round a central axis. Interiorly the stems are truly 



1 Sachs, p. 498. 



2 Sachs, p. 499. " ' 



