

CHAPTER XVI. 



SYMMETRY OF THE SPOROPHYTE. 



AMONG plants at large various types of symmetry are recognised. The 

 most simple form is the sphere, a type of symmetry without polarity, that 

 is, having no distinction of apex and base : it is the usual initial form of 

 the individual, when it is first delimited as the zygote or the spore. Occa- 

 sionally this form may be retained to maturity, as in the sporogonium of 

 Riccia : but in the vast majority of plants polarity appears early in the 

 individual life, usually with growth localised in relation to it. The body 

 thus produced may develop variously as regards an imaginary axis of 

 construction, which passes between the two poles. Three types of 

 symmetry are usually distinguished where polarity exists: (i) the radial 

 construction, where the development is equal in all directions round the 

 imaginary axis : (2) the bisymmetric or bilateral, in which the construction 

 is flattened equally on both sides : and (3) the dorsiventral, where the 

 construction is also flattened, but not equally on both sides, the result 

 being two faces which differ obviously from one another in form, and 

 usually also in inner structure. These types of symmetry may, as a rule,, 

 be related to the external conditions under which the parts are developed i 

 thus orthotropous, or vertical parts are almost always radial or bilateral - f 

 while plagiotropous, that is oblique or horizontal parts, are commonly 

 dorsiventral, or occasionally bilateral. Dorsiventrality of the shoot, where 

 it exists, has usually some evident relation to the external conditions of 

 life, such as the incidence of unequal lighting, or oblique disposition to 

 the action of gravity : and it may also be seen in lateral branches to be 

 connected with the relation of the part in question to the chief shoot 

 which bears it. In some cases it may arise from inner causes, 1 but 

 investigation has shown that dorsiventrality of the shoot is usually to be 

 referred to some external determining influence. 



1 A striking example of this is brought forward by Willis, in the Podostemaceae, 

 where dorsiventrality appears in erect and anemophilous flowers. He suggests that this 

 condition has been forced upon them, without reference to any advantage, by the steadily 

 increasing dorsiventrality of the vegetative system (Annals of Botany, 1902, p. 593). 



