THE DEVELOPMENT OF ORGANISMS 



781 



parthenogenetic ovum or a vegetative multiplication on the part 

 of the prothallus. An apogamous sporophyte has the haploid number 

 of chromosomes. 



But a gametophyte may be produced from a sporophyte without 

 any spore, either from an arrested sporangium or from vegetative 

 tissue. An aposporous gametophyte has the diploid number of 

 chromosomes. Fact and theory agree ! 



Fig. 131. 



Prothallus or Sexual Generation of a Fern ( Aspidium) . Under surface. 

 After Strasburger. AR, archegonia or female reproductive organs; AN, 

 antheridia or male reproductive organs; PR, lobed prothallus about the 

 size of half a threepenny bit; R, rhizoids. 



SEED-PLANTS OR SPERMATOPHYTES. — These are the 



higher plants, often called Phanerogams or Flowering Plants in 

 contrast to the Cryptogams or Flowerless Plants. The scientific 

 emphasis should certainly be on the seed, for that was the main 

 step in evolution. It implies that the embryo develops for some 

 time within a large megasporangium (the ovule), in dependence on 

 the parent plant. It is usual to divide the seed-plants into the 

 Gymnosperms, like pines, with the seeds exposed, and the Angio- 

 sperms, like ordinary Flowering Plants, with the seeds enclosed in 

 a case, badly called the ovary. 



A detailed comparison of a heterosporous Pteridophyte with a 

 typical Flowering Plant is given on the next page. 



While many puzzles remain, such as the contrasted sporophytes 

 of moss and fern, or the origin of the seed, or the peculiar endosperm 

 of ordinary Flowering Plants, there is no doubt that the recognition 

 of alternation of generations from Bryophyte (or earlier) to Flowering 

 Plants, in which it is so well disguised, has unified and elucidated 



