THE LIFE -HISTORY OF PLANTS. 



161 



advising those who have the opportunity to study 

 for themselves the remarkable variations and adapta- 

 tions of leaf -structure to the circumstances or " en- 

 vironment " under wnich they grow. In particular 

 the comparative study of the structure of the leaves 



the leaf is alike on both sides, and instead of the leaf 

 being placed more or less horizontally, so as to catch 

 the light on its upper surface, it is then often placed 

 as in Gladiolus, more or less vertically, so that both 

 sides get an equal chance of exposure. When, on the 



Fig. 48. —Floating and Submerged Leaves of Eammculus aquatilis. 



of epiphytal Orchids may 

 be commended to micro- 

 scopic observers, as not only 

 intrinsically interesting, but 

 as likely to lead to important 

 cultural suggestions. 



Speaking broadly, a leaf 

 consists of a flat plate of 

 cellular tissue, traversed by 

 a framework of wood-cells 

 and vessels, and covered on 

 all sides by a skin or epi- 

 derm. Very generally the 

 cells constituting the sub- 

 stance of the leaf are dif- 

 ferent in form and size in 

 different portions of the 

 same leaf. Thus those 



near the upper surface — the one most exposed 

 to the light — are more densely packed, with few or 

 no interspaces between them, and with more green 

 matter (chlorophyll) in their interior. The cells 

 near the lower surface of the leaf are usually more 

 irregular in shape, more loosely aggregated, and 

 with less green matter. When the leaf-structure is 

 uniform on both surfaces, or nearly so, the colour of 

 35 



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contrary, the colour of the 

 leaf is markedly very dif- 

 ferent on the upper and 

 lower surfaces respectively, 

 then it frequently happens 

 that the " chlorophyll " is 

 specially collected in a 

 special layer of cells just 

 beneath the upper epiderm, 

 and appropriately termed 

 "palisade" ceUs, from their 

 resemblance to park palings 

 (Fig. 49). 



Fig. 49. — section ol a Lieat, sflowmg tne ep 

 palisade, and branching Cells (magnified). 



The Framework of 

 the Leaf. — As regards 

 the framework of "veins," 

 or "ribs," or "nerves," it 

 is first of all necessary to disabuse the reader's mind 

 of any idea that these organs have anything more 

 than a remote analogy with the animal struc- 

 tures so named. They consist of wood-cells, which 

 serve to stiffen and uphold the blade of the leaf, 

 and of "vessels" of various kinds, which transmit 

 fluid and air to and from the stem and leaf. 

 The number and arrangement of these " ribs " 



