CHAPTER XVII 

 THE SPERMATOPHYTA 



This enormous group, to which the early portions of our text 

 were largely devoted, includes the most famihar and abundant 

 part of the earth's plant population. Directly or indirectly, 

 the seed plants furnish almost all the food supply of the human 

 race, all of its timber and fiber plants, and a great majority of 

 the animal and vegetable products which form the basis of our 

 civihzation. It is this division of the plant kingdom which is 

 present most intimately in the thoughts and lives of men, and 

 which for a long time provided practically the entire subject- 

 matter for the science of botany. 



In vegetative characters the spermatophytes are not remarkably 

 different from the pteridophytes. They display the same 

 vigorous development of root, stem, and leaf, and although they 

 commonly show a greater specialization and differentiation of 

 their tissues, particularly in the higher groups, the fundamental 

 plan established in the ferns, with its emphasis on a well devel- 

 oped fibro-vascular system, is retained and further developed. 

 Growth of the stem in diameter by means of an active cambium 

 occurs in most seed plants, although it is much reduced or 

 absent in the more delicate herbaceous species. 



The Origin of the Seed. — The distinguishing feature of the 

 spermatophytes, as their name indicates, is the development among 

 them of a new reproductive structure, the seed. In an earlier 

 chapter we have outlined briefly the evolution of the seed-habit 

 and its significance; but in view of the greater familiarity with 

 the lower plants which we now possess, it will perhaps be worth 

 while to describe the seed and its origin somewhat more fully 

 before we take up in detail the various plant groups in which 

 this structure occurs. 



Relation to Structures in the Lower Groups. — The seed-producing 

 habit is a direct development from such a condition of heterospory 

 as has been attained by the higher pteridophytes. It will be 

 remembered that in the water ferns, in Selaginella, and in Isoetes, 



344 



