So 



ANATOMY OF THE GYMNOSPERMS 



thick-walled cells occur in the genus Pinus to the extent of 56 per 

 cent, they show a diminishing frequency, eventually becoming rare 

 and are ultimately replaced by thin-walled cells. Reference to 

 them in the following diagnoses is always specified by (i). In 43.9 

 per cent of the genus the upper and lower walls are thin and 

 absolutely devoid of pits. For diagnostic purposes such cells are 

 always referred to as (2). In some cases they are so undeveloped 

 as to be obscure and readily broken out in the process of section 



FIG. 19. PINUS PALUSTRIS. Radial section of a medullary ray showing charac- 

 teristic pits on the lateral walls : a, a thin wall broken out ; b, thick-walled 

 parenchyma ; c, thin-walled parenchyma. X 280 



cutting, so that they are often entirely wanting (fig. 19, a-c). 

 Such thin-walled cells are typically developed in P. palustris, 

 P. taeda, etc., and it is to be observed that they are always asso- 

 ciated with the highest forms of development. Transition forms 

 occur. These are first seen in the soft pines, where occasional 

 thin-walled cells devoid of pits are interspersed and are often 

 conterminous with the thick-walled elements. In the hard pines 

 the same relation exists, but it is gradually reversed until the 



