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A TEXTBOOK OF THEORETICAL BOTANY 



seen with the naked eye in such species as Hypericum perforatum, which gets 

 it specific name from this circumstance (Fig. 1018). 



i^ Epidermis 



Oil gland 



X. Oil globule 



Leaf mesophyll 



3| 



Fig. 1018. — Myrtus communis. Transverse section of leaf 

 showing a large oil gland in the mesophyll. 



Sclereid cells, often of complex shape also occur, particularly in those 

 hard-leaved plants such as Camellia and Olea, which are called sclerophyllous. 



The orientation of these sclereids is usually definite in each case, lying either 

 transversely, like pillars across the mesophyll, or longitudinally, in the spongy 

 mesophyll or between the upper epidermis and the palisade layer (Fig. 449). 



The Petiole. 



In transverse section petioles are seldom perfectly circular, but are flattened 

 or grooved on the upper (adaxial) surface. The two rims of the groove 

 frequently expand gradually as the leaf blade is approached and pass insensibly 

 into the base of the lamina. Not infrequently the rims have the laminar 

 structure all the way down the petiole, which is thus fringed with assimilating 

 tissue, which may even continue some distance down the stem and is then 

 called decurrent (Fig. 1019). Where the leaf margin is toothed, the teeth may 

 also be continued along the petiolar fringes and in some plants, such as 

 Viburnum and Cerasus, the teeth on the petiole are transformed into one or 

 more extra-floral nectaries ; small, stalked, cup-shaped glands which pro- 

 duce nectar (See Fig. 1020). Other nectaries are produced on the leaves of 

 many species, usually on the under side of the blade or of the stipules, by local 

 modifications of the epidermis. The secretory surface layer in all cases is 

 called the epithem. The biological significance of such nectaries is doubtful. 

 The amount of nectar and its sugar content vary considerably. Sometimes 



