2S 



SYSTEM A TIC POMOJ.OCY 



inoilification lU'ai-csl to ^it-eii leaves are scuU-Uavcs whieli in 



fruits envelope the «rr()\vin^' i)oints of hiuls. These are ])roteetive 



sti-iictiiiTs. Tlie floi'al and l-epi'o- 

 (luetive organs of tiowers are a 

 little less like true leaves than 

 scales hut ai'e clearly modified 

 leaves. Tlie first leaves a plant 

 possesses, the seed-leaves or co- 

 tyledons, are modifications of fo- 

 liao:e-leaves. Spines and tendrils 

 may be modified leaves. Lastly, 

 leaves are not uncommonly stor- 

 age organs for food, as the fleshy 

 leaves of bulbs of which the onion 

 is an example. Other forms and 

 uses of leaves exist, but these are 

 commonest and most important 

 for the work in hand. 



45. The parts of a leaf. — A 

 typical leaf con- 

 sists of three 

 parts. The broad 

 green expanded 



portion is called the hlade or Umina (Fig. 15). 



The stalk by which the blade is attached to the 



stem is the leaf-stalk or petiole. Some leaves, 



as those of the apple and pear, have tw^o leaf- 

 like basal appendages called stipules; leaves 



possessing them are said to be stipulate, those 



without exstipulate. All foliar leaves have an 



upper surface and a lower one. The upper is 



exposed to the sun and is smoother and of a 



deeper green ; the lower surface is paler, more Fig. IG. Leaf of 



roughened by veins and is usually more pu- vefned; ^"ovate^ 



bescent. aP^^ acute; base 



acute ; margins 

 Leaves are almost endlessly diversified, and entire. 



yet within the limits of either species or va- 

 riety the characters of its parts are remarkably constant. It 

 follows that the leaf is a most valuable organ in making distinc- 



FiG. 15. Leaf of an apple, 

 blade ; b. petiole ; c. stipules 



