514 



BOTANY 



PART II 



the shoot originates at the base of the depression between them. 

 Monocotyledons, on the other hand, have a single large terminal 

 cotyledon, the growing point being situated laterally (fig. 479 a). 

 In both cases the root is formed from the end of the embryo which is directed 

 towards the micropyle ; its limits can be readily traced in older embryos, and it 

 can be seen that a few cells are derived from the hypophysis adjoining the 

 embryonic cell. After fertilisation a considerable accumulation of reserve materials 

 is necessai-y in the embryo-sac both for the development of the embryo and for its 

 future use. It is thus of importance that a means of transfer of these materials 

 should exist. In the simplest cases the endosperm simply enlarges, crushing the 

 surrounding tissues of the nucellus ; often the antipodal cells, which are the 

 structures which lie nearest to the cbalaza, are entrusted with the function of 

 nourishing the embryo-sac. They then increase in number and sometimes undergo 

 considerable further development. Other portions of the embryo-sac may grow 

 out as long haustoria which sometimes emerge from the micropyle and sometimes 



Fig. 479. — Stages in tlip development of the embryo of Cajisella bursa 

 pastoris (A-D). h, Hypopliysis ; et, suspensor ; c cotyledons ; p, 

 plumule. (Aft«r Hanstein, magnified.) 



Fig. 470 a. — Young embryo 

 of Alisma Plantago. c, 

 Cotyledon ; v, growing 

 point. (After Hanstein, 

 magnified.) 



penetrate the tissue beneath the chalaza. In some cases {Utricularia) a special 

 store of reserve material is laid up for transference to the macrospore. 



The phenomenon of apogamy (^^^), which has recently been accurately investi- 

 gated in several Angiosperms, must be considered here (cf. p. 283). Thorough 

 investigation of the embryology has shown that in these cases we have to do with 

 apogamy and not, as was first assumed, with parthenogenesis. The reduction in 

 number of the chromosomes (p. 86) is omitted in the development of the embryo- 

 sac, all the nuclei of vvliich have the vegetative number of chromosomes ; they are 

 diploid vegetative nuclei, not haploid generative, and the egg-cell is apogamous 

 and not parthenogenetic. The fact tliat the loss of sexuality is particularly shown 

 in polymorphic genera such as Alchemilla, Rieracium, Taraxaaim, Thalicirum, is 

 explained by Stra.sburgeu as resulting from the excessive mutation which has 

 occurred ; in time this, together with continuous crossing of the mutations in 

 various directions originating from the same species, would lead to a considerable 

 weakening of the sexuality, and from this the apogamous reproduction would follow. 

 On the other hand there are extraordinarily polymorphic genera such as Jlosa and 

 Ruhus which have only sexual reproduction. According to the investigations of 

 O.STENFELD and Rosenberg, the genus Eieracium is of special interest, since the 



