CHAPTER TWENTY-TWO 



SEED PLANTS: GYMNOSPERMS 



Seed plants form the most conspicuous part of the earth's 

 vegetation, and they include the majority of the plants that 

 are of interest as sources of food, lumber, and fibers. They 

 are the plants with which we are most familiar, and for this 

 reason the first seventeen chapters of this book have been de- 

 voted to a description of their structures, processes, and en- 

 vironmental relations. Here the great groups into which the 

 seed plants are divided will be described, and some of the 

 mqre important families in these larger groups will be briefly 

 discussed. The two most important groups of living seed 

 plants are the Gymnosperms and the Angios perms. 



The gymnosperms. The familiar representatives of the 

 gymnosperm group are the conifers, or cone-bearing trees. 

 The name "gymnosperm" (Greek: gymnos, naked, and 

 sperm, seed) suggests the most distinctive feature of the 

 group. The seeds are borne exposed on the upper surface of 

 scales, which make up the cones. They are not inclosed by 

 a pistil wall as are the seeds of -the next group. 



The conifers. The conifers have scale and needle leaves. 

 The red cedar and arbor vitas are of the scale-leaf type ; the 

 pines, spruces, and hemlocks are of the needle-leaf type. The 

 stems are woody, much branched, and of large size. Indeed, 

 the largest trees in the world, the giant sequoias, or Big Trees, 

 of California, belong to this group. The distinctive feature 

 of gymnosperm wood is the absence of water-conducting 

 tubes. The wood cells perform the double function of 

 supporting the tree and conducting the water. Many of 

 the conifers have large resin tubes extending throughout 

 the plant. Just what advantage comes to a plant through 



294 



