490 



LECTURE XXIX. 



this. If the leaf is divided or branched, then in like manner the halves of the entire 

 leaf situated right and left of the mid-rib are symmetrical or unsymmetrical : 

 nevertheless they are always equally and oppositely constructed, as our right and 

 left hand. Fig. 325 shows several foliage-leaves of an Aroid, the orthotropic and 

 radially constructed stalk of which grows vertically out of the earth, while its bilateral, 

 symmetrically branched lamina is at the same time dorsi-ventral, and thus situated 

 oblique or horizontal. 



Flat foliage-leaves, as every one knows, are differently constituted on the under 

 and upper sides — they are dorsi-ventral. The lower side is frequently hairy while the 

 upper is smooth : the veins project as ribs below, while on the upper side there are 

 generally corresponding depressions : the under side is dull, the upper side shining 

 and dark green, and so forth. The difference between the upper and lower sides 



appears still more conspicuous 

 on regarding the transverse 

 section of a leaf under the 

 microscope, as represented in 

 Fig. 326. If we suppose the 

 mid-rib MN bisected by a ver- 

 tical line, this corresponds to 

 the principal section of the 

 leaf, and divides the whole 

 bilateral lamina into a right 

 and a left half. We can ima- 

 gine no plane of division situ- 

 ated in any way at right angles 

 to this, however, by which the 

 transverse section of the leaf 

 would be so divided into equi- 

 valent upper and lower halves. 

 As the figure shows, the whole 

 organisation of the leaf is 

 difFerendy constituted in the 

 upper and lower halves, though 

 no very definite boundary line 

 exists between the two; and it is just in this that the character of the dorsi- 

 ventrality lies. It must be noticed, however, that the visible anatomical structure 

 which we here take into consideration, is itself again simply the expression of a 

 dorsi-ventral constitution situated still more deeply in the invisible organisation. 

 It can be shown that in every individual cell of the leaf the half turned upwards 

 is differently constituted and re-acts differently towards stimuli from the lower one. 

 This remarkable fact comes out still more clearly in the case of those plants which 

 consist not of cell-tissue but of simple vesicles : in such cases the dorsi-ventrality 

 is often not to be established by means of the microscope, but by the reactions 

 towards gravitation and light, which compel us to the assumption that in such 

 simple utricles the molecular structure presents corresponding relations of direction 

 across the axis of growth. 



