330 GENERAL BOTANY 



the living mesophyll cells of the leaf with the water-conducting 

 tissue of the main stem and its lateral branches. 



The root of the spruce and its relatives does not present any 

 new features that need be discussed in an elementary textbook. 



SUMMARY 



1. Two cambium layers are developed in the spruce, which enable 

 it to increase its stem in thickness and to form a protective outer 

 cork jacket which insures against too rapid changes in temperature, 

 loss of water, and the attacks of insects and fungi. In ferns this 

 cork jacket is unnecessary, since the stem is usually underground 

 and the outer skeletal layer, once formed, is in no danger of being 

 destroyed by the annual growth of the stem in diameter. 



2. The growth of the cambium forms a wide wood and phloem 

 cylinder for conducting the larger quantities of foods and water 

 made necessary by the greatly increased leaf exposure of the spruce 

 trees and their allies. 



3. Food storage is provided for in the wood rays of the central 

 cylinder instead of in the pith and cortex, as in the ferns. This 

 provision was made necessary in the higher plants when the wide 

 pith and cortex of the ferns was gradually eliminated as a result of 

 the secondary production of wood and phloem by the cambium. 



4. The leaf traces and gaps are present in the spruce, but they 

 become buried by the secondary products of the cambium. Their 

 presence in the spruce stem is an indication of the relationship of 

 two groups of plants which in other respects are widely separated. 



Asexual reproduction. The spruces are monoecious, bearing 

 both staminate and ovulate strobili on the same tree. Each stro- 

 bilus is a modified shoot, like the strobili of the lycopods and 

 cycads, with a central axis and lateral sporophylls arranged in a 

 spiral form. The staminate strobili terminate lateral shoots at 

 the ends of the main branches (Fig. 195, a), where they live 

 through the winter in the bud stage and first make their appear- 

 ance, in temperate climates, early in May. The microsporophylls 

 are scalelike, and each microsporophyll bears two microspo- 

 rangia on its lower abaxial surface (Fig. 197, e,/). A micro- 

 sporophyll with its two sporangia is commonly called a stamen, 



