300 MORPHOLOGY OF THE ANGIOSPERMS 



divisions. The egg is formed directly by the division of a spore nucleus 

 (the third division from the spore mother cell). The gamete could not 

 be foiTned earlier, unless the sterile tissue of the gametophyte were 

 eliminated; the spore would then become the egg directly. The number 

 of prothallial cells is seven in the Adoxa and Plumbago types; three, in 

 Plumbagella, where two are triploid fusion nuclei. The embryo sac 

 of the Adoxa type resembles that of the Polygonum type but is tetra- 

 sporic in origin, and the egg is formed two cell divisions earlier than in 

 the Polygonum type. 



The Fritillaria type is like the Plumbagella in the presence, at the 

 chalazal pole, of the sac of triploid nuclei, at the third division from 

 the mother cell — three haploid nuclei form two triploid nuclei. This 

 type is unlike the Plumbagella type, in that one more division (the 

 fourth division from the mother cell) occurs in the formation of the 

 egg. It resembles the Polygonum type superficially in its eight nuclei, 

 but four of these nuclei, the antipodals and one polar, are triploid. 



The tetrasporic Penaea, Drusa, and Peperomia types introduce a 16- 

 nucleate sac. The egg is formed at the fourth division from the spore 

 mother cell, the second from the spore, as in the Allium, Oenothera, and 

 Fritillaria types. The presence of sixteen nuclei in these sacs, in con- 

 trast to the eight of the Polygonum type — where one more division 

 occurs and only eight nuclei are formed — is the result of the four-spore 

 make-up of the sac. The four spores developing together form 16 nuclei 

 in two divisions, whereas one spore of the Polygonum type forms eight 

 in three divisions. 



In summary, the various subtypes show reduction in the development 

 of the mature gametophyte from five successive divisions to three; in 

 the formation of the egg from the spore, from three divisions to one; 

 in prothallial cells, from seven to three. The fifteen prothallial cells of 

 the 16-nucleate types suggest an increase, rather than a reduction, but 

 the additional cells belong, morphologically, to the additional spores 

 that contribute them. The large number of nuclei in these types is not 

 evidence that they are more primitive than the 8-nucleate types, as has 

 been suggested. 



The number, arrangement, and function of the nuclei and cells of 

 the embryo sac are remarkably constant for so great a group as the 

 angiosperms, which have many lines of specialization in other characters. 

 It seems doubtful that other major types will be found, even though 

 the embryo sac of only a small number of the many genera has been 

 studied. (Variations in apomictic genera have recently been cited as 

 possibly new types.) Evidence concerning the possible polyphyletic 

 origin of the angiosperms can probably not be found in the embryo 



