16 



BOTANY 



section ; those at right angles to it being termed cross or transverse 

 sections. Such parts of plants as may be divided by each of three or 

 more longitudinal planes into like halves are termed either Polysym- 



Fig. 13. — Diagram showing the so-called de- 

 cussate arrangement of leaves. 



Fig. 14. — Diagram showing two-ranked 

 alternate arrangement of leaves. 



B 



metrical, Radial, or Agtinomorphic. The degree of symmetry 



peculiar to any leafy shoot will be more 

 apparent from a diagram, that is if the 

 leaves which it bears be projected on a 

 plane at right angles to its axis. The 

 radial symmetry of a shoot with opposite 

 leaves is clearly shown in the adjoining 

 diagram (Fig. 13). A shoot with its 

 leaves arranged alternately in two rows 

 shows quite different relations of sym- 

 metry. The diagram of such a shoot 

 (Fig. 14) can only be divided into similar- 

 halves by two planes. When such a 

 condition exists, a member or plant is said 

 to be BISYMMETRICAL. When, however, a 

 division into two similar halves is only 

 possible in one plane,, the degree of sym- 

 metry is indicated by the terms sym- 

 metrical, MONOSYMMETRICAL, or ZYGO- 

 morphic. When the halves are equal, but 

 have a different structure and are spoken 

 of as ventral and dorsal sides, the body 

 is termed dorsiventral. Ordinary foli- 

 age-leaves exhibit this dorsiventral struc- 

 ture, and their upper and lower surfaces 

 are not only different in appearance but they also react differently 

 to external influences. In the accompanying figure (Fig. 15) such a 



,s- 



Fia. 15.— Diagram of a foliage-leaf. 

 A, Surface view ; B, transverse 

 section ; s, plane of symmetry. 



