452 GYMNOSPERMS LESS, xxxm 



which they become waterproof : this process, besides pro- 

 tecting the interior of the stem from external moisture, 

 prevents the access of nutrient matters to the epidermis 

 and outer layers of cortical parenchyma. Both these layers 

 consequently die and peel off, the outer surface coming to 

 be formed by the cork itself. 



The wood of pines contains no vessels, i.e., cells joined end 

 to end so as to form a continuous tube, but only tracheides, 

 z>., elongated spindle-shaped cells with lignified walls and 

 devoid of protoplasm (p. 417). Radial bands of cells 

 mostly parenchymatous, are formed between the tracheides 

 of the secondary wood, and give rise to the secondary 

 medullary rays (c, med. r) to which the radial striation of 

 the wood is due : they increase in number with the increase 

 in thickness of the wood. The tracheides formed in 

 autumn have smaller cavities and thicker walls than those 

 formed in spring and summer : hence the formation of 

 annual rings. The tracheides are not scalariform like those 

 of ferns, but their walls have at intervals circular depressions 

 perforated in the centre and called bordered pits. The 

 tracheides of the primary xylem bundles have spirally 

 thickened walls, like the spiral vessels of ferns. The 

 phloem, both primary and secondary, consists of sieve- 

 tubes and parenchyma. 



The growing point of Gymnosperms presents a striking 

 difference to that of ferns and other flowerless plants. It 

 consists simply of a mass of meristem cells among which no 

 apical cell is to be distinguished. 



Pines, like horsetails and club-mosses, reproduce by 

 means of cones or floivers. These are of two kinds, male 

 and female, so that sexual differentiation is carried a step 

 further than in Selaginella, in which sporangia of both sexes 



