THE PLANT KINGDOM 255 



Spermatophytes or seed plants, which are now the most successful 

 of all the higher plant types. The spore has several obvious dis- 

 advantages as a means of producing a new plant, owing to its 

 minute size. Among the lower plants these difficulties are parti- 

 ally overcome by the production of spores in huge quantities, but 

 the system of independent, free-living gametophytes, developed 

 from single-celled spores, is subject to many difficulties at best. 

 In the seed plants, as in a few of the most advanced Pteridophytes, 

 there are (as we have previously noted) two kinds of spores — 

 microspores and megaspores — which produce male and female 

 gametophytes, respectively. The happy innovation introduced 

 by these highest plants, however, was to retain the single mega- 

 spore within the sporangium, intimately attached to the mother 

 plant, where it germinates into a much reduced female gameto- 

 phyte. This whole structure, with the addition of a coat or 

 integument, is the ovule. Only a few ovules, in comparison with 

 the great number of spores formerly produced, are borne by the 

 plant. The microspores (pollen-grains) are still liberated into 

 the air in great numbers, just as among lower plants, but instead 

 of falling on the ground and germinating there, they are carried 

 to the ovule or near it, where each produces two male gametes, 

 one of which may fertilize an egg. 



Not only have the seed plants abolished the delicate, free- 

 living gametophytes, with all the consequent dangers and diffi- 

 culties in the process of reproduction, but they have also 

 established a much more successful method for insuring the 

 growth of the young plant. The fertilized egg grows at once 

 into the embryo, which draws the materials for its development 

 directly and abundantly from the mother plant, and is thus 

 relieved of the necessity of producing them by its own activity. 

 About the embryo is deposited this supply of concentrated food in 

 the form of endosperm. The growth of the embryo ceases after 

 a young root and one or two primitive leaves have been formed; 

 and embryo and endosperm, tightly enclosed in the integument of 

 the ovule which has now become very tough and strong, is known 

 as the seed. This becomes detached from the parent plant and 

 may remain in a dormant condition for a long time, sometimes 

 many years; but on the occurrence of favorable conditions it 

 will germinate and the embryo within it will begin to grow, 

 bursting its shell, al)S()r])iiig the stored food, sending forth roots 



