200 



BOTANY 



tART I 



the receptive oosphere results from spermatozoids, which have been 

 liberated into the surrounding water in an actively motile condition, 

 being chemotactically attracted by substances excreted from the 

 egg-cells. 



Numerous transitions between the two conditions show clearly 

 that oogamy has been derived phylogenetically from isogamy. From 

 this it follows that the antheridia and oogonia are homologous with 

 one another, and also with the sporangia (cf. also Fig. 231). 



It is not until after the entry of a spermatozoid that the egg-cell 

 becomes capable, either at once or after a resting period, of developing 



further. As a rule, after becom- 

 ing surrounded by a thick wall 

 it separates from the parent 

 plant as a unicellular oospore 

 (Fig. 230, 3 osp), and only com- 

 mences its independent develop 

 ment later with the bursting of 

 its wall. In other cases, while 

 still attached to the parent plant, 



FIG. 231. Diagrams founded on Algae. sp,\ Spor- . , . . \ J 



angium with spores; a, antheridium with sperma- it develops into a multicellular 



tozoids; o/, oogonium with several and o,, with and more or less segmented 



a single egg-cell ; o, the pore in the cell wall. body which produces unice ll u l ar 



asexual spores ; these spores are 

 then set free from the parent plant as the true reproductive bodies. 



Bryophyta, Pteridophyta. Oogamy is the rule in these groups. 

 The male and female sexual organs are of more complicated structure 

 than in the Thallophyta. They are not single cells but have walls 

 composed of a layer of sterile cells. In the case of the multicellular 

 antheridia (Fig. 232, 1) this encloses a larger or smaller number of 

 cells with abundant protoplasm (the SPERMATOGENOUS TISSUE), from 

 each of which a spermatozoid will be formed. In the flask-shaped 

 female sexual organ, which is known as an ARCHEGONIUM, there is 

 only one egg-cell surrounded by the wall formed of a layer of cells 

 (Fig. 232, 2). The archegonia and antheridia are homologous 

 structures. They have special arrangements for opening at maturity. 

 In the Bryophyta they are borne on the thalloid or leafy gametophyte. 

 In the Pteridophyta the sexual organs are not borne on the leafy 

 plant (which has been seen to be reproduced by spores) but on the 

 prothallium, which is the sexual generation or gametophyte living 

 independently of the sporophyte. 



In the Bryophyta and Pteridophyta the oosphere after fertilisation, 

 which takes place in the same way as in the Thallophyta, develops 

 forthwith into the EMBRYO which becomes the SPOROPHYTE (the 

 stalked capsule in Bryophyta, and the leafy plant in the Pteridophyta). 



Spermatophyta. In this group also the sexual reproduction is 

 exclusively oogamous, but the sexual organs have come to differ 



