DIV. 1 



MORPHOLOGY 



201 



widely from the simpler types. The gametes are formed in greatly 

 reduced or unrecognisable archegonia and antheridia ; these are pro- 

 duced in extremely reduced prothallia, often consisting of only a 

 few cells, that are enclosed in the pollen grains and ovules of the 

 flowers. The pollen grains contain the male sexual cells, 

 while one or more egg-cells are contained in the ovule. 



The peculiar method of fertilisation in spermato- 

 phytes is connected with the fact that the egg-cell 

 remains enclosed within the ovule in the flower. The 

 pollen grains after being shed from the pollen sac require 

 to be carried to the ovules in the case of Gymnosperms, 

 or to a special receptive portion of the ovary called 

 the STIGMA in the Angiosperms. 

 This is the process of POLLINATION. 

 Most of the manifold modifications 

 of the flowers of Angiosperms are 

 adaptations to the method of pollina- 

 tion ("), which always involves special 

 means of transport of the pollen. 

 When, as is often the case, male 

 and female organs are present in 

 the same flower, i.e. in hermaphrodite 

 flowers, it might be assumed that no 

 special arrangements would be neces- 

 sary to bring the pollen to the stigma. 

 More accurate investigation has, how- 

 ever, shown that such adaptations 

 exist in abundance and are often of 

 the most detailed nature. They do 



, . , FIG. 232. 1. Antheridium, with wall of sterile 



mply aim at the conveyance cells enclosing the spermatogenous tissue. 



2. Archegonium, with corresponding wall 



and an egg-cell. Both based on a Liverwort. 



of the pollen to the stigma of the 

 same flower ; often they render such 

 SELF-POLLINATION (autogamy) 

 impossible and effect CROSS-POLLINATION (allogamy), i.e. the con- 

 veyance of pollen to the stigma of another flower on the same plant 

 (geitonogamy) or on another individual (xenogamy). The transport 

 of the pollen may be by wind, water, or the agency of animals 

 attracted to the flowers by their colour, scent, or nectar ; thus most 

 flowers can be classed as ANEMOPHILOUS, HYDROPHILOUS, or ZOIDIO- 

 PHILOUS (cf. the Special Part). Most spermatophytes have thus 

 become independent of the presence of water for the purpose of 

 fertilisation and are in a special sense land plants. 



In addition to plants which show allogamy there are others which have arrange- 

 ments leading to autogamy, either when cross-pollination does not succeed, or 

 primarily as in cleistogamous flowers (cf. the Special Part). 



After pollination the pollen grain grows out into a POLLEN TUBE, 



