DICOTYLEDONS 203 



shows nothing peculiar. The pollen-spores are gener- 

 ally of the same tetrahedral type found in the lowest 

 Archegoniates, and we see that even in these highest of 

 all plants the microspores have hardly departed from 

 the primitive type found in the lowest liverworts, the 

 division of the spore mother-cell and the structure of 

 the ripe spores being identical in both. The germina- 

 tion of the microspores and fertilization are as in Mono- 

 cotyledons. 



The development of the embryo follows at first much 

 the same course as in Monocotyledons, but very early 

 there is in most cases a marked difference manifested. 

 In the Monocotyledons, as a rule, the apex of the 

 embryo becomes transformed into the single cotyledon, 

 the stem-apex being formed laterally ; but in typical 

 Dicotyledons the apex of the embryo forms the stem- 

 apex, while the two opposite cotyledons are developed 

 secondarily as lateral appendages of it. It may be stated, 

 however, that Monocotyledons are known in which the 

 stem is derived from a portion of the apex of the young 

 embryo, and it is possible that a similar condition may 

 obtain in some of the lower Dicotyledons. At present 

 our knowledge of the embryogeny of the lower mem- 

 bers of both of the great divisions of Angiosperms is 

 far from complete. 



The degree of development attained by the embryo 

 before the seed ripens, varies a good deal in different 

 Dicotyledons. In some forms, especially saprophytes 

 and parasites with minute seeds, e.g. the Indian pipe 

 (Monotropa), the embryo in the ripe seed is completely 

 un differentiated and consists of a few cells only. 

 Usually, however, it is well developed, and the primary 



