216 APPLIED BIOLOGY 



inclosing the ovules; but in gymnosperm cones the spore- 

 leaves do not unite to inclose the ovules, and hence these 

 ovules and the resulting seeds are said to be " naked." 



Such a cone containing spore-cases or ovules on its spore- 

 leaves corresponds in its function to a flower containing pistils 

 only. Within each ovule is formed an egg-cell from which 

 an embryo develops after fertilization.* This is effected by 

 a cell which enters through a tube (pollen-tube) from a 

 pollen-grain. In short, fertilization is very similar to that 

 described for plants with true flowers ( 189). 



Pollination of conifers is due to gravity and the wind, the 

 pollen falling on the spore-leaves and sliding down to the 

 ovules. The pollen-grains in pine forests are often so abun- 

 dant that they cover the ground, the so-called " sulphur snow." 

 The grains are interesting objects for miscroscopic study, be- 

 cause they have two " wings," which are really air-sacs. 



After fertilization is accomplished the fertilized egg-cell 

 in an ovule develops into an embryo, the surrounding cells 

 of the ovule develop into endosperm stored with food, and 

 the outermost cells form the hard seed-coats. When the 

 seeds are fully developed (sometimes two years are required) 

 the leaves of the cone dry and separate so that the seeds may 

 fall out. The seeds of some species of conifers have wings 

 which assist in scattering by the wind. 



FRUITS OF SEED-PLANTS 



212. The word fruit in popular usage refers to various 

 edible products of flowers, but in botanical terminology it 

 means the structure, usually containing seeds, which develops 

 from a flower, chiefly from the ovary. In the simple cases 

 one ovule forms one seed, and the wall of the ovary forms the 

 surrounding structure. These may be in part edible (as in 



* In some gymnosperms there are two or more egg-cells in each ovule 

 (as shown in Fig. 61) ; but only one survives and forms the embryo in the 

 which develops from one ovule. 



