100 BOTANY. [chap. hi. 



text-books — " a flattened-out expansion of the stem ;" 

 and this is in reality what the typical leaf originally 

 is, an expansion of a plate of tissue containing all 

 the important elements of the stem, to which it is 

 attached by a broad base, with the fibro-vascular bundles, 

 or veins, sunk in the parenchymatous portion, running 

 in a parallel series and not forming a network; and 

 when the leaves grow erect, as in the grasses, irises, etc., 

 the two surfaces agree in structure ; in fact, it may be 

 said that the utility of the leaf was thoroughly realised 

 by monocotyledonous plants, and the fundamental idea 

 thoroughly developed, whereas in the dicotyledons those 

 improvements and finishing touches were evolved that, 

 surface area being equal, enables the perfectly- evolved 

 leaf of the Dicotyledon to do far more work than its 

 primitive monocotyledonous prototype. 



In the dicotyledonous leaf there is often a more or 

 less elongated stalk or petiole that serves to carry the 

 leaf away from the branch producing it and placing it 

 under favourable conditions with regard to light. As a 

 rule, the leaves are placed horizontally, and exhibit 

 a bilateral structure ; that is, having the two sides diffe- 

 rently organized, as already explained, thus introducing 

 a division of labour not met with in the leaves of the 

 majority of Monocotyledons. The fibro-vascular bundles 

 are well developed, the larger branches being connected 

 by innumerable small veinlets, the whole forming a 

 complicated network best seen in the so-called " skeleton 

 leaf." This arrangement of the veins insures a supply 

 of material for assimilative purposes to every part of the 

 leaf, and also at once removes the assimilated material 

 from the leaf to be utilized at once in other parts of the 



