30 ANATOMY OF THE GYMNOSPERMS 



elements two important factors are involved. Owing to the 

 peculiar conditions of growth attending their organization, they 

 are formed under a minimum tension, in consequence of which 

 they rapidly attain to relatively great size, and it is therefore 

 found that the first tissue of the season is always most open. 

 But this feature depends again upon the second factor. In con- 

 sequence of the great excess of nutrition supplied during this 

 period of growth, and the very rapid process of construction 

 which follows, secondary growth of the walls is limited and these 

 structures remain thin, while the lumens are correspondingly 

 large. In transverse section such cells almost invariably show a 

 greater length of the radial diameter, which is never less than 

 the tangential. As growth advances there is a slight but pro- 

 gressive change whereby the secondary development of the wall 

 increases somewhat and becomes constantly thicker ; but the 

 principal alteration occurs in the form of the cell in such manner 

 that the radial and tangential diameters tend to equality. Such 

 thin-walled structure is developed during the first four or six 

 weeks of growth, and it is therefore designated as spring wood. 

 Toward the end of the general growth period, which has elsewhere 

 been shown to terminate by the second or third week of July for 

 about ninety per cent of trees and shrubs in this latitude (80), 

 the structural character of the ring is subject to more or less 

 profound change as the rate of growth diminishes and the inter- 

 nal tension of the tissues increases. The isodiametric form of 

 the tracheid is then replaced by an extension of the tangential 

 diameter and a shortening of the radial diameter, the latter pro- 

 gressing more rapidly than the former. This alteration is of such 

 a nature that in those tracheids which represent the last product 

 of the season's growth the opposite tangential walls are closely 

 approximated or even in actual contact, while the tangential 

 dimension is sensibly increased as compared with tracheids of 

 the same row in the spring wood. Coincident with these changes 

 the secondary wall acquires an unusual prominence, and in many 

 of the hard pines, as also in Pseudotsuga, it becomes so exces- 

 sively thickened that the lumen is reduced to small dimensions 



