350 



PRACTICAL COURSE IN BOTANY 



different shapes, and in some species is branching and filamen- 

 tous, like the protonema of a moss. Generally, however, it 

 is flat and more or less two-lobed, as shown in Fig. 501. It 

 is small and inconspicuous and very short-lived, being of 

 importance only in connection with the work of reproduction. 

 Look with your lens for a cluster of small, bottle-shaped 

 bodies just below the deep cleft in the heart. If you can- 

 not make out what they are, put a thin section through 

 a part of the prothallium containing one under the micro- 

 scope, and you will see that they are the archegonia. Lower 

 down among the rhizoids, near the pointed base, will be 

 found the antheridia. In some species the prothalli are 

 dioecious, one kind bearing antheridia, the other archegonia, 

 but this is rare among the true ferns. 



408. Fertilization. - - This process is the same in all essen- 

 tials as in the bryophytes. As in other cryptogams, it can 



take place only under 

 water, - - a circumstance 

 which points to an aquatic 

 origin for this subkingdom, 

 and through them to the 

 entire flora of the globe. 

 The archegonia differ 

 somewhat in shape from 

 those of the liverworts and 

 mosses, but a section under 

 the microscope will show 

 that they consist of essen- 



FiQ.503.-Youngarchegoniumof.afem, tially the Same parts. On 

 magnified: K, neck canal cell; K', ventral aCCOUnt of the similarity of 



canal cell ; O, egg cell. , -, , i . -, 



these organs, the pterido- 



phytes and bryophytes are often classed together as Arche- 

 goniates. 



409. Alternation of generations. Among the section of 

 ferns that we have been considering, the order of alternation 

 corresponds in all essentials to that prevailing among the 



