APPENDIX. 



473 



gential section cuts across them. This will explain the difference in 

 the appearance of the medullary rays in the two sections. The differ- 

 ences in the appearance of the bordered pits are due to the fact that 

 the tracheides are four-sided, two of the sides being approximately 

 radial and two tangential, and that the bordered pits are confined to 

 the radial walls. Thus, in radial section, the radial walls are not cut 



ALBUMINOUS 

 CELLS 



TRACHEIDAL 

 / CELLS 



STARCH CELLS 



PITS ON 

 TRACHEIDES 



CAMBIUM 



SIEVE PLATE 

 Fig. 12. Radial Longitudinal Section of Stem of Pine. 

 The section is taken at the junction of secondary wood and phloem. 



through and the pits are seen in surface view ; while in tangential 

 section, the radial walls are cut through and the pits are seen in section 

 (Fig. 13). 



Fig. 14 shows a transverse section of the foliage-leaf. The 

 epidermis consists of extremely thick-walled cells with a 

 strong cuticle. Owing to the erect position of the leaf stomata 

 are developed all over its surface. The guard- cells are sunk 

 beneath the level of the epidermis, so that there is an outer 

 cavity leading down to the stoma. Beneath the epidermis 

 there is a fibrous sclerenchymatous hypodermis interrupted 

 beneath the stomata. 



The parenchymatous mesophyll consists of thin- walled 

 cells, whose walls show numerous peg- like infoldings of cellu- 



