I] LEAF-MOSAIC 9 



frequent, as in Screw Pines, and are probably often con- 

 cerned in causing apparent alterations of phyllotaxy, e.g. in 

 Aloes, Draccma, some Aroidece, &c. 



We shall have occasion to discuss these matters from 

 a more general point of view later on. 



Meanwhile the student must be on his guard against 

 confounding phyllotaxy, and the displacements referred 

 to above, with the position of the leaf- surfaces them- 

 selves. Phyllotaxy is, strictly speaking, concerned with 

 the positions of the leaf-insertions ; but several causes 

 may co-operate in bringing about curvatures of various 

 kinds which place the leaf -surf aces in directions not 

 at all obviously related to the phyllotaxy. 



This subject of the arrangement of leaf-surfaces has 

 come to be referred to in text-books under the head of 

 leaf mosaic. If we look down vertically from above at a 

 freely growing erect foliage branch of the Sycamore, or 

 the Norway Maple, for instance, it will be seen that the 

 leaf-blades are so arranged that they catch the incident 

 solar rays over their whole upper surfaces, and still do 

 not seriously obstruct the access of such rays to leaves 

 lower down : this is brought about partly by the decussate 

 phyllotaxy, and partly by the larger size and longer stalks 

 of the lower leaves which carry the blades of the latter 

 well to the periphery of the area on which we can project 

 the foliage (Fig. 1). 



If we compare with this vertical branch a horizontal 

 one of the same tree, the examination shows that a similar 

 object the advantageous display of as much of each 

 leaf-surface as possible is brought about by the twisting 

 of the leaf-stalks, so as to bring those which would 

 approach the vertical if they grew out from the leaf- 

 insertions in the ordinary direction, more into a horizontal 

 plane (Fig. 2). 



