3.] 



CHAPTER I. GENERAL MORPHOLOGY. 



13 



member are mainly inherent ; but it has been ascertained in many 

 cases that external conditions have a preponderating influence, as 

 is well shown in the preceding instances of a change of symmetry 

 due to the intensity and direction of the incident rays of light, or 

 (as in certain dorsiventral flowers) to the action of gravity. 



When a body or a member cannot be symmetrically divided into 

 two similar halves in any plane whatever, it is said to be asymmetric. 

 The asymmetry in these cases is frequently associated with dorsi- 

 ventrality ; as in some Mushrooms (e.g. Lenzites abietina) ; in 

 some foliage-leaves which are oblique, 

 that is, the right and left halves of 

 which are not symmetrical (e.g. Elm, 

 some Begonias) ; and in some flowers 

 (e.g. Aconitum, Delphinium). 



3. The Development of the 

 Body and of the Members. It 

 is explained in a subsequent para- 

 graph dealing with Reproduction, 

 that new organisms are developed 

 either from what are termed vegetative 

 reproductive organs, or from specialized 

 reproductive cells termed spores: the 

 latter case only is now considered. 



There are certain important varia- 

 tions in the mode of development, of 

 the body from the spore. As a rule, 

 the whole spore takes part in the de- 

 velopment of the body ; whereas in a 

 few plants (Characese, Gymnosperms 

 generally) only a portion of the spore 

 is actively concerned in the process. 

 In the former case the embryogeny is 

 said to be holoblastic ; in the latter 

 meroblastic. 



Again, the spore gives rise, as a 

 rule, to an embryo which presents a 

 general similarity to the adult form into which it gradually and 

 directly developes : such an embryogeny is direct or homoblastic. 

 But in certain cases the embryo produced by the spore differs 

 more or less widely from the adult form, and does not directly de- 

 velope into it, but bears it as a lateral outgrowth ; this mode of 



FIG. 5. Charafragilis (after Prings- 

 heim, x4). Heteroblastic embryo- 

 geny : ap, apical portion of shoot of 

 the embryo ; r, primary root of em- 

 bryo, springing from the [oospore ; 

 sr, secondary roots ; I, leaves amongst 

 which lies the growing-point of the 

 adult shoot. 



