HIGHER LAND PLANTS 



183 



Figure 11.19 Floating and mosquito ferns. Salvinia, o floating tern: 

 1, habit. Azotla, o mosquito fern; 2, natural size; 3, enlarged; 4, 

 enlorged leaf. (Used by permission, from Morphology of Vascular 

 Plants; Lower Groups, Psilophytales to Filicales by Arthur J. Eomes. 

 Copyright 1936. N. Y.; McGrow-Hill Book Co.) 



tinctive male gametophytes (pollen grains) which are 

 shed, and larger spores into a distinctive female game- 

 tophyte; both gametophytes develop within their 

 spores and complete their final development in the 

 larger female spore; shed pollen reaches the female 

 gametophyte, sperm fertilizes egg or eggs, the zygote 

 develops into an early embryo which soon stops de- 

 velopment and enters a resting stage; the resting 

 embryo plus other tissues form a structure called a 

 seed (Figure 1 1.20). 



Seed production: The female sporangium produces a 

 single female spore which develops into the female 

 gametophyte; female sex organs may or may not de- 



velop; within the female gametophyte an egg or eggs 

 forms; the egg is replaced by an ovule composed of 

 egg, sporangium wall (called nucellus), and one or 

 two surrounding layers of tissue (integuments); in- 

 teguments have a small opening (micropyle) at either 

 the sporangium stalk or opposite end of the ovule; 

 through this opening fertilization can occur. 



The male sporangium (called a pollen sac) produces 

 many small spores, each developing into a resting 

 stage of a male gametophyte (a pollen grain, com- 

 posed of inner gametophyte surrounded by an outer 

 protective wall, wall is the spore wall and is usually 

 two-layered); the pollen grain wall often has variable 

 but distinctive shapes and markings; in a pollen grain 

 the male gametophyte without first differentiating 

 into a sex organ produces sperm; later and upon con- 

 tact of pollen grain and ovule, the pollen grain de- 

 velops an outgrowth, or pollen tube, which digests 

 its way through the intervening ovule tissues and 

 deposits the sperm. 



In seed development fertilization occurs, a zygote 

 results, and the embryo grows slightly. Then the 

 embryoenters a brief to prolonged resting stage, which 

 lasts until germination. Finally, the embryo sporo- 

 phyte, female gametophyte, and ovule (three genera- 

 tions) form the seed. Seed formation is essentially 

 completed when the integuments become the protec- 

 tive outer layer, or seed coat, of the seed. Seeds either 

 germinate immediately, the apparent primitive an- 

 cestral condition, or assume a resting stage prior to 

 germination. In some cases germination starts only 

 after certain environmental influences. 



Classification Note: As mentioned earlier, biologists 

 once classified spermatophytes as a phylum distin- 

 guished from the pteridophytes. It was also men- 

 tioned previously why spermatophytes and pterido- 

 phytes are no longer usually considered natural 

 groupings of plants. In this older system of classifica- 

 tion two classes, the Gymnospermae and Angio- 

 spermae, also were recognized. Although none of 

 these groups now tend to be recognized in the older 

 sense, tracheophytes are regularly labeled as "pteri- 

 dophytes" or "spermatophytes" and spermatophytes 

 as "gymnosperms" or "angiosperms." It will be seen 

 that the term "angiosperm" still applies to a natural 

 group but not one equivalent to gymnosperms. 



The gymnosperms include all seed plants except 

 the Class Angiospermae, or flowering plants. They 



