THE PLANT: ITS STRUCTURE, LIFE - PROCESSES AND ENVIRONMENT 



17 



faster than another, so that the tip circles in 

 the same direction as the hands of a watch, to the 

 right, or with the sun (as in the hop), or in the 

 opposite direction (as in the morning-glory). 

 Such plants unwind and reverse their direction if 

 placed upside down, and they will not twine on a 

 horizontal or nearly horizontal support. 



The flower. — When the work of root, stem and 

 leaf has stored a suiBcient surplus of food, the 

 plant proceeds to flower. The century plant spends 

 many years in this process ; trees usually take 

 four or five years or more ; biennials, as the beet 

 and turnip, require two, while annuals complete 



their preparation in a 

 few days or weeks. 

 The food is stored in 

 roots, tubers, root- 

 stocks, stems, or in 

 modified leaves such 

 as we find in bulbs. 

 In the latter case, the 

 fully formed minia- 

 ,ture flowers can often 

 be seen on cutting 

 open the bulb. 



The flower is usu- 

 ally spoken of as a 

 modified branch. In 

 their early stages, 

 flower - buds are so 

 much like leaf -buds 

 that they cannot be 

 told apart. But the 

 growing leaf-bud pro- 

 duces leaves that soon 

 become separated by the elongation of the stem, 

 while in the flower-bud they remain crowded to- 

 gether, and become modified into dissimilar struc- 

 tures. 



The outermost set of these structures, the calyx 

 (Fig. 14), consists of green, leaf-like sepals whose 

 function is to protect the internal parts, much 

 as the outer leaves of a bud protect the innermost. 

 Next comes the corolla, consisting of petals, which 

 are leaf-like except in color. Instead of chloro- 

 phyll they possess a number of pigments that are 

 either held in solution in the cell sap or appear in 

 solid form. These, by their combination, produce 

 an endless variety of coloration. The appearance 

 of white petals is due (like that of snow) to the 

 presence of air. 



The next set of organs, the stamens, often have 

 a leaf-like basal part, while the upper part pro- 

 duces an anther, i. e., a structure consisting usually 

 of four cavities filled with pollen-grains or micro- 

 spores. At maturity these cavities open and dis- 

 charge their pollen in the form of yellow dust. The 

 innermost set of organs, known as the carpels, are 

 often leaf - like, as, for example, in the pea - pod, 

 whose texture, color and veining are essentially 

 those of a leaf. It corresponds to a leaf folded 

 lengthwise on the midrib, so as to bring the edges 

 together. Along the united edges are borne the 

 seeds. Such a carpel is called a simple pistil or 

 ovary ; when several are united, as usually is the 



B2 



Fig. 40. Flower of fuchsia in 

 longitudional section. The 

 ovary is at a. 



case, the resulting structure is called a compound 

 pistil or ovary. The term ovary is applied to the 

 part that contains the seeds or ovules, while the 

 term pistil includes also the style and 

 stigma. The stigma is borne on the 

 summit of the ovary, often on a stalk 

 called the style, and consists of a 

 sticky or hairy surface designed to 

 catch and retain the pollen. 



Inside the ovary are found the rudi- 

 mentary seeds, or ovules (Fig. 40). An 

 ovule usually has two coats, inside of 

 which is a mass of tissue called the 

 nucellus, containing a cavity called the 

 embryo-sac, or macrospore. Inside of 

 this are found one or more eggs. 



In order that the egg may develop, 

 pollen must be brought to the stigma 

 and there germinate (Fig. 41), sending 

 out a long germ tube that makes its 

 way down the style to the embryo-sac. 

 A nucleus (Fig. 42, pn) makes its way 

 from the pollen-tube through an open- {^^^) 

 ing that is formed at its end, and en- 

 ters the embryo-sac, where it unites with the 

 nucleus of the egg (e). This constitutes the act of 

 fertilization, and the characters of both parents 

 are thereby united in the nucleus so formed. This 

 nucleus is called the fertilized egg. Since each of 

 the fusing nuclei has the same number of chromo- 

 somes, the fertilized egg has twice as many, and 

 this double number is found in all the cells of the 

 plant that develop from the fertilized egg, until, in 

 the mother-cells, that give rise to the pollen-grains 

 and embryo-sac, the number is 

 suddenly reduced to one-half. 



The fertilized egg soon be- 

 gins to develop and eventu- 

 ally forms a tiny plant with 

 rudimentary root, stem and 

 leaf, as we find it in the seed. 

 The coats of the seed develop 

 from those of the ovule ; some- 

 times the ovary wall or a part 

 of it remains permanently at- 

 tached to the seed. The endo- 

 sperm of the seed comes from 

 two endosperm nuclei (Fig. 42, 

 end), which fuse with a nuc- 

 leus from the pollen-tube 

 (s pn). The endosperm may 

 thus show the characters of 

 both parents. In corn, in 

 which the endosperm deter- 

 mines the color of the grain, an 

 ear of yellow corn that re- 

 ceives pollen partly from yel- 

 low and partly from blue corn 

 may show, on the same ear, 

 both blue and yellow grains 

 side by side. 



Since pollen is easily in- 

 jured by rain or dew, various 

 devices exist for keeping it 

 dry. The closing or drooping 



Fig. 42. Embryo-sac of 

 a lily. Showing: the 

 union of the nucleus 

 from the pollen- 

 tube C»n), with the 

 egg (e): the second 

 pollen- tube nucleus 

 lspn)j unites ■with 

 two endosperm pro- 

 nuclei {end), which 

 multiply and form 

 the endosperm: 

 antipodal cells 

 (ant), nurse nuclei 

 which help nourish 

 the egg, etc., (nr), 

 pollen-tube (p). 



