Britton and Taylor : Life H 



young sections ; these cells, including the epidermis, are filled with 

 starch. The epidermis and cortex are often invaded by fungal 

 hyphae. 



The vascular bundle is concentric ; the xylem portion is en- 



* 



veloped in the phloem. The central cylinder is surrounded by a 

 well-defined endodermis and phloem sheath, the radial walls of 

 which are thin and fragile ; the phloem elements are represented 

 by two or three imperfect rows of narrow parenchymatous cells 

 and sieve tubes ; the xylem consists of scalariform tracheids with 

 occasionally small spiral tracheids close to the phloem. The 

 medullary parenchyma is composed of large, thick-walled cells, 

 pitted, and early showing sclerosis, but not as early as the funda- 

 mental tissue outside the bundle. 



Sterile Leaves 



The sterile leaves are linear, slender and tortuous. The de- 

 velopment of the leaf is very slow, the lower portions having long 

 been fully formed while the apex is still unfolding. The vernation 

 is circinate (Fig. 79). Owing to the more rapid growth of the cells 

 on the dorsal side than those on the ventral, the leaf is rolled up 

 on the ventral side. When fully developed they bear on their 

 dorsal side two rows of stomata alternating with three rows of 

 glands (Fig. 81), sometimes four or more rows of glands. The 

 glands seem to originate from special cells cut off from the epi- 

 dermis ; these epidermal cells frequently do not lengthen, keeping 

 very nearly an isodiametric shape ; when they do lengthen the 

 glands remain at or near the upper wall. 



The young leaves and the tip of the stem are more or less 

 completely clothed with trichomes early turning brown. These 

 are not to be confounded with the glandular hairs. They are 

 composed of two or more cells and are extremely long, measur- 

 ing in some instances 1 mm. to 3 mm. or perhaps more. The longest 

 glands of the leaf measured nearly 100 /i, others j6 fi and in 

 width 3 1 fi. Some are composed of one cell, others of two cells ; 

 they are all club-shaped and contain granular protoplasm. These 

 glands were rarely found on the ventral surface, and sometimes 

 they did not appear to follow any law as to their formation on the 

 dorsal surface, though, for the most part, they were formed in al- 

 ternate rows with the stomata. 



