812 GENERAL PHYSIOLOGY. 
that bears them, and the toothing does not arise from cells 
specially adapted for that purpose on the edge of the leaf, as 
Mercklin has supposed. The hairs on the under surface of the 
leaf are also formed from below upwards. 
‘Leaves developed centripetally (called also the basilar or 
basipetal mode of leaf formation) are more numerous than the 
preceding, and this method may be well studied in the formation 
of the leaves of the Hyacinth; of this sort are the leaves of 
* Sanguisorba officinalis, Rosa arvensis, Cephalarva procera, ce. 
In them the terminal leaflet is first produced, and the others 
appear in successive pairs downwards from apex to base. The 
stipules are produced before the lower leaflets. All digitate 
leaves, and those with radiating venation, belong to the cen- 
tripetal mode of formation as regards their digitate venation. 
‘In some plants, as Acer, the two preceding modes of de- 
velopment are combined. This is called mixed formation. In 
Acer platanoides the lobes and the midribs of the radiating lobes 
form from above downwards, the lower lobes being produced 
last, but the secondary venations and toothings are developed 
like those of the Lime-tree. In Monocotyledons we meet with 
the parallel (included by some writers with the basilar) leaf - 4 
formation of Trécul. All the veins are formed in a parallel 
manner, the sheath appearing first. The leaf lengthens espe- 
cially by the base of the blade, or that of the petiole when 
present. 
‘Leaves furnished with sheaths, or having their lower por- 
tions protected by other organs, grow most by their base ; while 
those which have the whole petiole early exposed to the air grow 
much more towards the upper part of the petiole.’ 
CHAPTER 2. 
GENERAL PHYSIOLOGY, OR LIFE OF THE WHOLE PLANT. 
Havine now briefly treated of the special functions of the 
elementary structures and of the organs of nutrition, as such 
structures are alone intimately concerned in maintaining the 
life of the plant and its various organs, we proceed to give a 
sketch of general physiology, or the whole plant in a state of 
life or action. In doing so, we shall first notice the substances 
required as food by plants and their sources ; then proceed to 
consider the function of absorption, or that process by which 
food is taken up dissolved in water ; and lastly, show how this 
fluid food is distributed through the plant, and altered in the 
leaves, so as to be adapted for the development of new tissues, 
