OF BACCHARIS GENISTELLOIDES 693 
tropic reaction. The stem had turned towards the light, the bend- 
ing region extending to about 3 cm. from the tip. The two wings 
nearest the aperture had turned at right angles to each other and 
in the direction of the rays, the third wing had moved to a position 
just behind and parallel to one of the others. In this way only its 
very edge was illuminated. Although at first it seemed as if the 
wings must have played a positive part in the movement towards 
the light, probably a purely mechanical explanation is to be as- 
signed for their position. By experiment it was found that these 
would be the positions assumed by any rigid objects —such as 
strips of cardboard — fastened to a bending axis, so that probably 
only the stem was directly concerned in the reaction. 
‘THE LEAF 
The leaves themselves vary in size. The largest found measured 
2.2 x 1.3 cm., the smallest 5 x 3 mm. (f 3). They appear, as 
above pointed out, at the basal portion, but also occasionally on 
new shoots, developing from buds on these regions. A transverse 
section of the leaf (/ zo) shows that here also we have a fairly 
compact mesophyl structure. The intercellular spaces are, as in 
the wings, comparatively small. In the leaf, however, sharply 
distinguishing it from the wings in structure, we find a noticeable 
dorsi-ventral differentiation ; for not only has the leaf a definite 
palisade parenchyma of two layers of cells, but a difference is also 
to be observed in the number of stomata present on the two sur- 
faces. There are 106 stomata to the square millimeter on the 
lower surface to 50 on the upper. It is to be noted as significant 
that the whole number of stomata per square millimeter would aver- 
age almost exactly the same in the case of the wing and the leaf. 
REVERSION TO THE JUVENILE LEAF-FORM 
Several experiments were started with a view to inducing the 
re-formation on adult shoots of the juvenile leaf-form. Some of 
the growing shoots were kept in a moist chamber, others in a dry 
and still others in a dark chamber for several weeks, but no results 
were obtained. This may have been due to one or more of several 
Causes. First, of course, these particular agents may not induce 
leaf-formation in this plant; or again the failure may have been 
