PHYLLOTAXY 
47 
upon horizontal shoots, which would obviously be incon- 
venienced by strict adhesion to the radial arrangement suited 
to the erect shoot. Sometimes the dorsiventrality is attained 
by the twisting of the leaf-stalks from the positions in which 
they arose, but more commonly there is a more or less two- 
ranked ( distichous ) phyllotaxy, the leaves arising upon the 
sides of the axis, and merely having to twist at their bases 
to place themselves horizontally. A further consequence of 
this is that the axillary branches also stand much in one 
plane. Good examples may be seen in the yew (Taxus), 
lime (Tilia), Betulaceae, Abies, Pinus, Ulmus, Anona, &c. 
In branches thus obliquely placed, a phenomenon termed 
anisophylly may often be observed. The leaves borne on 
the under side of the branch are larger than those on the 
upper, while the lateral leaves are intermediate in this 
respect. The difference is especially well seen in plants with 
opposite leaves. The phenomenon is largely dependent on 
external stimuli — gravity, light, &c. — but is hereditary or 
habitual in a number of plants, e.g. Centradenia, many 
Melastomaceae, Strobilanthes, Columnea, Tabernaemontana, 
Gardenia, Philadelphus, Salvia, Sambucus, Ligustrum, &C . 1 
In other plants, again, alterations of phyllotaxy occur for 
which no explanation can at present be given ; e.g. in Sola- 
naceae, Thelygonum, Quisqualis, Eucalyptus, Baptisia, &c. 
The Leaf in most cases is a thin green expanded organ 
borne on the stem, and performing the great functions of 
assimilation and transpiration, together with respiration and 
more or less storage of materials. Details as to internal 
structure must be sought elsewhere ; here it will suffice to 
say that the interior is usually made up of a spongy mass of 
green cells (the mesophyll ), with intercellular air-spaces be- 
tween them. These communicate with the outside air 
through minute openings ( stomata ) in the outer covering 
layer of cells or epidermis. The passage of gases for assimi- 
lation and respiration is through the stomata, whereas water 
also evaporates through the epidermis, the outer walls of 
which are usually covered by a waxy or corky layer, the 
cuticle . The cuticle is more or less resistent to the passage of 
1 Wiesner, Anisomorphie d. Pfl ., Sitzb. k. Akad. Wiei^ Cl. 1892; 
Stud. u. d. Anisophyllie tropischer Geivachse , l.c. CIII. 1894; Goebel, 
Organography of Plants, p. 65. 
