SPERM ATOPHYTES 271 



sac. In this case the endosperm functions as an intermediary between 

 the perisperm and the embryo, obtaining the food stored in the former 

 and passing it on to the latter. 



Xenia. — The phenomenon known asxeniais the appearance in the seed 

 of characters belonging to the pollen parent, when the pollen is foreign 

 (belonging to another race). For example, when a race of white or 

 yellow corn is crossed with pollen from a race of red corn, many of the 

 resulting kernels are red or mottled. It is found that this color belongs 

 to the endosperm, and that it is introduced by the male cell that enters 

 into the triple fusion. This means that in this case the endosperm is a 

 hybrid as well as the embryo. 



Embryo 



The embryo of angiosperms does not begin with free nuclear division, 

 as in gymnosperms (Tumboa and Gnetum excepted), but the first divi- 

 sion is accompanied by a wall. As the most fundamental difference 

 between dicotyledons and monocotyledons is found in the embryo, the 

 two groups must be considered separately. 



Dicotyledons. — The embryo of Capsella (shepherd's purse) is most 

 commonly used as a representative of the dicotyledonous embryo. The 

 egg divides transversely and subsequent tranverse divisions result in a 

 filament of varying length (fig. 600). This filament is the proembryo, 

 which later becomes differentiated into suspensor and embryo. The 

 terminal cell of the proembryo divides into octants (fig. 601), the four 

 terminal octants forming the stem and cotyledons, the four basal octants 

 forming the hypocotyl except its tip. In the octant stage the dermatogen 

 (the layer that produces the epidermis) is cut off by periclinal walls (fig. 

 602). In the interior the two other body regions are soon outlined, the 

 periblem (producing the cortex) and the plerome (producing the stele) 

 (fig. 603). 



At the tip of the hypocotyl the plerome is complete, but the peri- 

 blem and dermatogen are incomplete (fig. 603). This gap in the tip 

 of the hypocotyl is filled by the adjacent (second) cell of the pro- 

 embryo as follows: This cell divides transversely, and the daughter cell 

 next to the embryo is the hypophysis, which fills out the hypocotyl. The 

 hypophysis divides transversely, the inner cells completing the periblem, 

 and the outer cells completing the dermatogen (figs. 603-608). The 

 second cell of the proembryo, therefore, contributes both to the embryo 

 and to the suspensor, and the boundary between these two regions is 



