208 



PLANT STEDCTUKES 



active and even divide and form a considerable amount of 

 tissue, which usually nourishes the embryo until endosperm 

 tissue is developed, and then becomes disorganized; or 

 even invades the tissue of the nucellus. 



114. Development of embryo. — While the endosperm is 

 forming, the oospore has germinated and the sporophyte 

 embryo is developing (Fig. 180). Usually a suspensor, more 

 or less distinct, but never so prominent as in Gymnosperms, 



is formed ; at the end of it the 

 embryo is developed (Fig. 181), 

 which, when completed, is more 

 or less surrounded by nourish- 

 ing endosperm (Fig. 183). 



The two groups of Angio- 

 sperms differ widely in the struc- 

 ture of the embryo. In Mono- 

 cotyledons the axis of the em- 

 bryo develops the root-tip at one 

 end and the "seed-leaf" (coty- 

 ledon) at the other, the stem-tip 

 arising from the side of the axis 

 as a lateral member (Fig. 182). 

 This relation of organs recalls 

 the embryo of Isoetes (see § 90). 

 Naturally there can be but one 

 cotyledon under such circum- 

 stances, and the group has been 

 named Monocotyledons. 



In Dicotyledons the axis of 

 the embryo develops the root-tip at one end and the stem- 

 tip at the other, the cotyledons (usually two) appearing as 

 a pair of opposite lateral members on either side of the 

 stem-tip (Fig. 181). This recalls the relation of parts in 

 the embryo of Selaginella (see § 89). As the cotyledons 

 are lateral members their number may vary. In Gymno- 

 sperms, whose embryos are of this type, there are often 



Fig. 180. Curved embryo-sac of 

 arrowhead (Sagittaria), show- 

 ing in the upper right end a 

 young embryo, in the other 

 end the antipodal cells cut off 

 by a partition, and scattered 

 through the sac a few free en- 

 dosperm cells. — After Sciiaff- 

 ner. 



