518 



BOTANY 



Thnlictrum, is explained by STRASBUR<;I:K as resulting from the 

 excessive mutation which lias occurred ; in time this, together with continuous 

 crossing of the mutations in various directions originating from the sanir spec-jo. 

 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 Rosa and Jliiliiix which have only sexual reproduction. 

 According to the most recent investigations of OSTENFELD and HI >si N i.i in ;. i he genus 

 Hieraeium is of special interest, since the formation of the embryo within the <vuh; 

 may commence in very various ways. In most a tetrad formation accompanied by 

 a reduction division takes place, but only some of these ovules are found to have a 

 normal embryo-sac capable of fertilisation ; as a rule this is displaced by a vege- 

 tative cell which develops into an aposporous embryo-sac. In exceptional cases 

 apogamous embryo-sacs are formed. Thus in ///V/v/c////// apugamy and ap<isp<>r\ 

 are found in addition to the normal mode of development of the embryo as a result 

 of fertilisation. It is noteworthy that these plants of Hirr/n-him in the course of 



FIG. 505. Stages in the development of the emliryo of Capsxlln 

 bvrsa pastoris (A-I>). //. hypophysis : ft. suspc-n^,! : ,-. ,-,,t\- 

 ledons ; p, plumule. (After HANSTEIN, ma^iiili.-ci. ) 



Fl(i. :~>OC>. Yonnn eiiiln vo nl 



". C.Coty- 

 r, LM'owinx l*>iiit 

 (Alter HANS I I IN. 



fled.) 



generations may, after a larger or smaller number of apogamous or aposporous 

 embryos have been produced, give rise to descendants which exhibit normal 

 fertilisation. CLEISTOGAMY ( llb ) may be briefly referred to here, since GOEBKI. has 

 recently succeeded in reducing it to the same point of view. Cleistogamous flowers 

 which frequently occur in linpaticns, Lamium, Viola, etc. are to be regarded as 

 arrested formations, in which, however, the sexual organs attain maturity. They 

 result in all cases from insufficient nutrition whether due to want of light or to an 

 insuflicient supply of mineral food materials. Since such cleistogamous flowers 

 regularly set seed, the plants possessing them can succeed even though their 

 normal chasmogamous flowers are often sterile. Reference may also be made to 

 the various and often striking provisions which ensure the efficient nutrition of 

 the embryo and endosperm. While in the simplest cases the endosperm increases 

 greatly in size and gradually displaces the nucellus, the antipodal cells as lying 

 next the chalaza are often specially concerned in the nutrition of the embryo-sac. 

 In such cases they increase in number, and sometimes attain a large sixe. Other 

 portions of the embryo-sac itself may grow out as long haustoria which sometimes 

 emerge from the micropyle and sometimes penetrate into the chalazal region and 

 the surrounding tissue ; the abundant reserve materials which in some cases 



