INTERNAL 8TMUCTUBE OF LEAVES. 155 



of the Virginia Creeper (Fig. 130, c) may also be studied with 

 profit, and it is curious to see how tlie positions of some of the 

 leaves are altered by the fact that the bud is a double one. 

 The bud of the Indian corn (Fig. 130, d) shows that the same 

 law holds in the Monocotyledons as in the Dicotyledons. 



§ V. The Inteenal Structure oe Leaves. 



201. — The internal structure of leaves varies considerably. 

 In all cases, however, the leaf is composed mainly of thin- 

 walled, chlorophyll-bearing parenchyma, and this is to be re- 

 garded as the proper leaf tissue. The fibro-vascular bundles 

 constitute little more than the framework of the leaf and 

 its connection with the 

 stem, while the epider- 

 mis is hero, as elsewhere 

 in the plant, a covering 

 tissue. In the related 

 members of the plant, 

 such as bracts, scales, 

 floral envelopes, and 

 other phyllome struc- 

 tures, chlorophyll-bear- 

 ing parenchyma is gen- 

 erally wanting, but 



from true leaves it is pig. 131 —vertical section of a portion of the leaf 



■navolTr oTTov aVioonf Tlio oi Echinocitstis lobata. fl. epidermis of tlie uppftr 



rareiy ever aosent. xne g^^ace ; e', epidermis of the lower surface ; /the 



shape of the leaf, its Pf™ncfiyma constituting the" pallsade" tissue; 



-t^ . , ' ^, the looseandirregularparcnchymaof the lower 

 size, position, and re.- part of the leaf, in a part of the section the chlo- 



, ^ ' rophyll granules are shown, X 250. — ^From a 



latlOn to other mem- drawing By J. C. Aithur. 



bers, all have somewhat to do with securing the best disposi- 

 tion of the essential leaf tissue. 



202. — In leaves composed of one layer of cells, as in many 

 mosses and some ferns, obviously there is no need of any 

 special arrangement of the cells in order to secure their best 

 exposure to light, heat, gases, etc. In thick leaves, however, 

 the internal cells are clearly not so well situated as the 

 external ones are, hence we find such leaves possessing some 

 peculiarities in their structure which obviate this diflBculty. 

 Instead of being composed of solid tissues, their cells are 



