2.] 



CHAPTER I. GENERAL MORPHOLOGY. 



11 



to the parent stem : the upper or inner surface is here the ventral, the outer 

 or loiver, the dorsal. 



The difference between the dorsal and ventral halves may be 

 exhibited in their external form. Thus, the dorsal and ventral 

 halves of many fruits (Peach, Fig. 2 G ; or a pea-pod) may be 

 distinguished at once by their form. Or the difference may be 

 in the nature of the members which they bear (Fig. 3) ; thus, 

 creeping dorsiventral shoots commonly bear (adventitious) roots or 

 root-hairs on their ventral, and branches or leaves on their dorsal 

 surface ; or the one surface may bear lateral members, and the 

 other none. Or, finally, the difference may exist in their internal 

 structure; thus, in dorsiventral foliage-leaves, the internal structure 

 of the dorsal half is different from that of the ventral half. 



It must not be overlooked 

 that the terms radial, isobi- 

 lateral, and dorsiventral, may 

 be all applicable to one and 

 the same body or member, 

 according to the particular 

 feature winch is taken into 

 consideration. For example, 

 a branch of the Silver Fir is, 

 in its general appearance, 

 dorsiventral ; a dorsal and a 

 ventral half are readily dis- 

 tinguishable. But, since tho 

 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 im- 

 portant to distinguish clearly 

 between the symmetry of any 

 part of the body as a whole, and that of its constituent members. 

 Thus in many isobilateral and dorsiventral shoots, the stem, 

 regarded by itself, is radially symmetrical ; 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 symmetry of a body or of a member may change in the 



FIG. 3. Polysiphonia (Herposiphonia) (after 

 Naegeli). To illustrate dorsiventrality. The 

 horizontal stem bears the leaves (f) on the 

 dorsal surface ; the root-hairs (r) on the ventral 

 surface; and the branches (a b c) on the flanks. 



