112 



BOTANY 



The bundles are surrounded by parenchymatous sheaths, which 

 are composed of a number of layers of cells in the thicker nerves but 

 of a single layer only in the finer branches. The cells of these sheaths 

 are as a rule elongated and have no intercellular spaces. Strands of 

 sclerenchymatous fibres are frequently present on one or both faces of 

 the bundle (Fig. 129, 1), especially on the phloem side. Here, in the 

 case of the larger bundles, the strand of sclerenchyma is curved ; in 

 cross-section it occupies the projection of the rib to the under side, and 

 serves to give rigidity against bending to the lamina. In some leaves 

 strands of sclerenchyma also occur between the bundles (Fig 129, 1) 

 and also at the leaf-margin. Such sclerenchymatous or collenchymatous 

 strengthenings of the margin are protective against shearing forces 

 that would tend to tear the lamina (Fig. 129, 2). Large leaf-blades 



FIG. 129. Leaf of Phormium tenax. 1. Transverse section ; Sc, plates and strands of sclerenchyma ; 

 A, green assimilatory parenchyma ; H, hypoderma serving for water-storage ; W, colourless 

 mesophyll (internal water-storage tissue). 2. Edge of the same leaf; E, thick brown epidermis ; 

 R, marginal strand of sclerenchyma fibres. (After NOLL.) 



which lack such marginal protection are torn by the wind (e.g. the 

 Banana). 



(b) EPIDERMIS AND MESOPHYLL. The foliage leaf is bounded on 

 all sides by a typical epidermis. In this, especially on the under side, 

 there are numerous stomata, while on the upper side they are often 

 absent (e.g. in almost all deciduous trees). 



On the under side there are on the average 100-300 stomata to the square 

 millimetre, but in some cases more than 700 may occur. Isolateral leaves as a 

 rule have stomata on both sides and floating leaves only on the upper surface. 



The tissue of the leaf-blade between the upper and lower epidermis 

 in the intervals between the ribs consists mainly of parenchyma and 

 goes by the name of MESOPHYLL. The finer veins are embedded in 

 it. Beneath the upper epidermis (Fig. 130 ep) come, as a rule, one 

 to three layers of cylindrical parenchymatous cells elongated at right 

 angles to the surface. These are called PALISADE CELLS (Fig. 130 pi), 

 contain abundant chlorophyll, and have intercellular spaces between 

 them. They constitute an assimilatory parenchyma. The cells often 

 converge below in groups (Fig. 130) towards enlarged collecting 

 cells (s). 



In the leaves of many trees, e.g. the Copper Beech, differences in the thickness 

 of the palisade layer are met with, its depth being much less in the " shade-leaves " 



