FERTILIZATION. 635 



germinal corpuscles (fig. 612, A, e v) into the germinal cell (fig. 



In Orc/iix, two of the corpuscles are sometimes developed into embryos. 

 In Citrus, as may be readily observed in Orange-pips, two embryos are 

 very frequently formed in the seed. 



Development of the Embryo. The development of the germinal 

 cell into the embryo exhibits some variations in different cases. 

 Most frequently the cell divides transversely, and the upper cell 

 often elongates (sometimes dividing again by septa) so as to form 

 a tubular confervoid filament, proembryo or suspensor, hanging 

 from the top of the embryo-sac, and bearing at its lower end the 

 true embryonal cell, which soon divides transversely and lengthwise 

 into a more or less globose mass of cells, which are ultimately 

 shaped into a mono- or dicotyledonous embryo. This suspensor is 

 seen especially in Crueifene, Scrophulariaceso, &c. ; it is a single 

 globular cell in Potamogeton. In Zea, Fritillaria, &c. the germinal 

 cell does not elongate at all. In Ordiis the suspensor grows out 

 from the micropyle. 



Changes in the Ovule during Fertilization. Different changes 

 are undergone by the parts of the ovule during the development of 

 the embryo (iig. b'13). In aperisperinic seeds the embryo in its 

 growth destroys all trace of the nucleus, and in the ripe seed lies 

 immediately within the coats. In seeds with endosperm the com- 

 monest condition is for the embryo- sac to become filled with 

 cells which are moulded over the embryo internally, and to expand 

 externally until the surrounding tissue of the nucleus disappears, 

 or remains only as an element in the coats of the seed. The tissue 

 developed in the embryo-sac forms the endosperm. In Piperaiva-. 

 \vmpha)acea3, and a few other cases a double perisperm is formed, 

 endosperm being formed both inside and outside the embryo-sac, 

 the latter being developed from the tissue of the nucleus. 



No rules can be given for the homologies of the "coats" of the seed 

 the testa and tcymvn or endopleura, which are formed either from lh< 

 priininc and *v< -untlhtc, or from these and 1 he nttcfi-ux -ami sometimes from 

 one alone of them, its tissues undergoing a different development in dif- 

 ferent layers. 



Fertilization. 



The existence of distinct sexes in plants was inferred by Linnaeus 

 from certain arrangements which he described, and which would fa- 

 vour the process of fertilization, though it was soon seen that in many 

 instances, as in the case of bisexual plants, the agency of the \\ ind 

 or of insects was required to convey the pollen to the stigma. Ex- 

 cept in the instances just alluded to, it was the general opinion that 

 sell -fertilization was the rule in hermaphrodite flowers, i.e. that 



