DEVELOPMENT OF PLANTS 



331 



branches are often formed. The growth of the Pinales is usually 

 prolonged and vigorous, the age of some of the giant trees of 

 California being estimated at over 5,000 years, and they may 

 attain the height of over 300 feet and a diameter of 30 feet. The 

 elongation of the stem is effected by a group of actively dividing 

 cells at the apex, while the increase of girth is brought about 

 by the activities of a cambium, as in the dicotyledons (see page 

 75). In fact, all the features seen in the cross-section of the 

 stem as the cork, cortex, phloem, cambium, xylem with its an- 

 nual rings of growth and medullary rays and the pith are sugges- 

 tive of the dicotyledons, save that the tissues are more simple. 

 This is particularly noticeable in the xylem, which is composed 



FIG. 250. A, radial section of the xylem of Pinus t, tracheids; p, border 

 pores or thin places in cell wall to promote transfer of fluids; m, medul- 

 lary ray of two cells accompanied by tracheids, mt. B, tangential section, 

 showing medullary rays, one cell broad and from three to nine cells high; 

 p, bordered pores. C, cross-section of the outer cells of pine leaf, showing 

 heavy epidermal cells, e; well-developed cuticle, c; sunken stoma, s; 

 strengthening stereome cells, st, and chlorenchyma, m. 



of very regularly formed tracheids with peculiar bordered pores 

 on their radial walls to promote the rapid transfer of fluids to 

 and from the medullary rays (Fig. 250, A, B). The character- 

 istic uniform and even-grain of pine, spruce, etc., is due to the 

 simplicity of the tracheids which compose the wood. This struc- 



