THE LEAF 



79 



of a number of them shows some with an elongated form, others with a median 

 constriction, and others again grouped in pairs as though resulting from a 

 recent fission (see Fig. 76, B, p. 115) : but it is doubtful whether it can be 

 said that plastids in general arise only by fission of pre-existing plastids. 



The structure of the palisade-parenchyma, with its oblong cells, 

 is admirably suited to accommodate large numbers of the chloro- 

 plasts, which thus present for the most part their edges to the light 

 incident upon the upper surface of the leaf. The cells of the spongy 

 parenchyma, with their irregular form, are less specially fitted for 

 this, and usually they contain fewer chloroplasts than those of the 

 palisade. They are also less important in that they get most of 

 their light at second hand, that is, after it has passed through the 

 palisade tissue. About midway between the upper and lower epidermis 

 the smaller vascular strands of the reticulum may be found traversing 

 the mesophyll, and in intimate relation with it (Figs. 47-49). They 

 vary in complexity from considerable bundles downwards, to a 

 minimum size where the xylem may be represented by a single tracheid. 

 The sieve-tubes stop short before this point is reached ; but certain 

 richly protoplasmic cells, like large companion cells, extend further 

 than they towards the vein-endings. Each strand is surrounded by 

 a parenchymatous sheath, the cells of which are in contact with the 

 cells of the mesophyll. Thus the conducting system penetrates and 

 thoroughly permeates the green tissues of the lamina. 



The characteristic function of nutrition carried on by the leaf is 

 discussed more fully in Chapter VIII. Here it may be stated simply 

 that in the presence of light the cells containing the green pigment 

 (chlorophyll) build up new organic compounds from carbon dioxide 

 obtained from the atmosphere, and water. The process is known 

 as Photosynthesis ; it is of fundamental importance to the 

 plant. 



In the leaves of many plants the period of this activity is limited by 

 the season, and autumn with its lowering temperatures and shortening 

 days leads to the fall of the leaf. At the base of the petiole a band of 

 corky tissue is formed in a transverse plane. Immediately above it 

 the cells become rounded off by increase of the intercellular spaces. 

 This is called the absciss layer , because the line of fracture is determined 

 by the weakness of its cells, and it is here that the leaf falls away. 

 The scar is covered by the layer of protective cork, while the vessels 

 and sieve-tubes running up to its surface are constricted by the pressure 

 of the adjoining cells (Figs. 6, 55). Thus the fall of the leaf causes 

 no open exposure of the living tissues liable to attack by intrusive 



