* 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 
undifferentiated and consists of «a few cells only. 
Usually, however, it is well developed, and the primary 
