8 PART I. MORPHOLOGY. [ & 



general appearance, dorsiventral ; a dorsal and a ventral half are 

 readily distinguishable. But, since the leaves are arranged sym- 

 metrically around it, it is in this respect radial. Again, since the 

 lateral branches arise right and left upon its flanks, it is in this 

 respect isobilateral. Hence it is important to distinguish clearly 

 between the symmetry of any part of the body as a ivhole, and 

 that of its constituent members. Thus in many isobilateral and 

 dorsiventral shoots, the stem, regarded by itself, is radially sym- 

 metrical ; the isobilaterality or dorsiventrality of the shoot being, 

 in these cases, indicated only by the mode of arrangement of the 

 leaves upon the stem. 



The causes which determine the symmetry of the body or of a 

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

 cases that external conditions have a preponderating influence, 

 such as the intensity and direction of the incident rays of light, or 

 (as in certain dorsiventral flowers) 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 asymme- 

 tric. The asymmetry in these cases is frequently associated with 

 dorsiventrality ; as in some Mushrooms (e.g. Lcnzites 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). 



5. The Development of the Body and of the Mem- 

 bers. The body, consisting of the primary members, is developed 

 from the spore. It is not proposed to enter now into the some- 

 what complicated details of the various modes of embryogeny 

 occurring in the different groups of plants ; but rather to consider 

 the development of the secondary members, that is, of those mem- 

 bers which are produced directly or indirectly by the primary 

 members. 



Whilst the plant is still an embryo, the whole of its protoplasm 

 is capable of growth, and is said to be in the embryonic condition. 

 As the development of the embryo into the plant proceeds, most of 

 the protoplasm passes over into the adult state, and is no longer 

 capable of growth. Certain portions of it, however, retain the 

 embryonic condition and properties, remaining capable of growth : 

 these portions of the protoplasm are termed growing-points, and 

 may persist throughout the whole life of the plant. Such growing- 

 points usually, but not exclusively, exist at the tip or apex of the 

 prinmry stem and of each of its branches, as also at the apex of 



