64 MINUTE MARVELS OF NATURE 



transparent cells, completes the structure, as 

 shown in the illustration — excepting only the 

 few darker rings of cells intermixed with the 

 palisade and spongy cells, which represent the 

 cut ends of the nerves or leaf veins. 



Fig. 40 gives the central vein or mid-rib from 

 the same section of this leaf showing that the 

 mid-rib gradually assumes a structure more 

 identical with that of the stem (a description of 

 which was given in the previous chapter) rather 

 than the leaf-blade. Figs. 41 and 42 illustrate 

 sections from the blade and mid-rib of the sun- 

 flower leaf for comparison ; but you will notice 

 that the rough leaf of the sunflower differs from 

 the smooth one of the laurel in having minute 

 many-celled hciirs arising from the epidermal 

 tissue. 



It is the continuation of the leal-stalk or 

 "petiole," as botanists term it, which consti- 

 tutes the mid-rib, and the same structure becomes 

 similar to the young stem as it nears it ; but 

 towards the apex of the leaf the various vascular 

 tissues otten disappear by degrees, merging their 

 original character in the more simple cellular 

 structure of the leaf 



If we examine the growing-point of a stem 

 where new leaves are being formed, to trace their 

 origin, we find at the apex (see Fig. 43) a conical 



