THE CASTOR BEAN 



115 



Minute Structure of the Leaf. A section across the petiole 

 of a leaf shows a structure similar to that found in the stem 

 of a plant, except that there is no regular ring of fibrovascular 

 bundles and no cambium layer. In this petiole may be seen 

 several fibrovascular bundles separated from each other; and 

 if these are traced down to the stem from which the petiole 

 of the leaf arises, they will be found continuous with the fibro- 

 vascular bundles of the stem. Followed into the blade of the 

 leaf, these bundles are found to pass out into it and form the 

 veins. Thus the veins of the leaf are simply an extension of 

 a few of the fibrovascular bundles that come from the stem. 

 Being hard and tough, they give sufficient rigidity to the leaf 

 to support the softer parts, which are the active portions of 

 the leaf structure. 



Microscopic Structure of the Blade. A cross section through 

 the blade of the leaf is most instructive, since it is in the blade 



st 



ep~ 



fb 



m st 

 FIG. 57. CROSS SECTION OF A BIT OF THE BLADE OF A LEAF 



cl, chlorophyll bodies; 



ep, epidermis; 



fb, fibrovascular bundles; 



m, mesophyll cells; 

 p, palisade cells; 

 st, the stomata. 



of the leaf that the most important function of plant life is 

 carried on. Upon its upper and under surface there are single 

 layers of cells, the epidermis; Fig. 57 ep. These are made of 

 small, irregular cells, closely compacted together and possessing 



