REPRODUCTION— THE FLOWER. 



57 



plumage of birds, and so the dissemination is 

 effected (figs. 70, 71). 



The chemical changes which occur during the 

 ripening of the fruit are varied and complex. 



*sd**d 





Tig. 70.— Seeds showing the Outer Skin or Testa with rugged prominences 

 or projections. The sections show the Seeds cut lengthwise, and show 

 the Embryo with its two Cotyledons, and the Kadicle surrounded "by 

 the Perisperm. 



All the time that it remains green it has the 

 same functions as other green members, but 

 afterwards in succulent fruits starch is converted 

 into sugar, various acids and other compounds 

 are formed which confer flavour on the fruit, 

 the explanation of which pertains to chemistry. 



Eig. 71. —Seeds with the Testa or Skin winged, or provided with a 

 tuft of Hairs, the object in each case being to secure the disper- 

 sal of the Seed. 



The same conditions as for growth generally 

 influence the formation of the fruit, but a larger 

 amount of heat and a fuller exposure to the sun 

 are required for the full development of the 

 fruit than for the leaves; hence it is that many 

 tropical plants can be made to grow here though 

 not to produce flower or fruit. 



The whole art of forcing depends on the 

 recognition of the facts just stated. 



Seeds, Germination. — As the fruit consists 

 of the ovary or ovaries of the flower in a mature 

 state, so the seeds are the ovules ripened in 

 consequence of fertilization (see p. 53). The 

 coats of the ovule become the coats of the seed, 

 sometimes dried up, at other times rendered 

 succulent. The nucellus frequently disappears 

 as the embryo grows, or it persists and supplies 

 food for the seedling plant. When it remains 

 it is called the perisperm or albumen, and a seed 

 like that of Wheat, which is provided with such 

 tissue, is spoken of as albuminous or perispermic, 

 in contradistinction to an exalbuminous or aperi- 

 spermic seed, as in the Pea or the Almond. The 

 embryo-sac also gets filled with a similar tissue, 

 which is called the endosperm, produced from the 

 fusion of the two "polar nuclei", and this also 

 may or may not be used up as the embryo plant 

 grows. The number of seeds produced, and 

 their form, vary greatly in different plants, 

 these variations being largely connected with 

 the preservation, protection, or dispersion of 

 the seed. 



When the seed with its contained embryo or 

 seedling is placed under propitious circumstances 

 growth commences, the seed, or, more correctly 

 speaking, the seedling begins to germinate, the 

 process having been previously described, see 

 p. 32. The circumstances propitious to ger- 

 mination are: sufficient moisture, an adequate 

 amount of heat, and access of oxygen. W^hen 

 these conditions act conjointly the seed- coat 

 swells, the cells of the embryo plant multiply, 

 chemical changes occur which result in the trans- 

 formation and solution of the food-stuffs stored 

 up in the perisperm or it may be in the tissue of 

 the seedling plant itself. In some cases, perhaps 

 all, the transformation of inert into available 

 matter is effected by an enzyme or ferment 

 secreted in the tissues of the seedling, so that 

 the growth of the embryo is the consequence of 

 changes analogous to those which occur in the 

 digestion of nitrogenous matters by the leaves 

 of the so-called insectivorous plants, and to 

 those which effect the digestion of starchy and 

 other matters in the stomach and intestines of 

 animals and of mankind. That there should be 

 any analogy between the growth of a seedling 

 plant and the digestion of our own food is not 

 apparent on the surface, but it is, nevertheless, 

 strictly true, and it is only one of many illus- 

 trations showing that the life of plants is, in 

 essence, identical with the life of animals and 

 with our own. The elementary construction is 



