Page:The New International Encyclopædia 1st ed. v. 17.djvu/55

HEPBODUCTION. 39 REPRODUCTION. (cliromosonies) in the newly foriiiod nucleus of the feitilizi'd egg, exactly one-half of the number are furnished by the egg nucleus and the other half by the sperm nucleus. These chromosomes are regarded as the twarers of heredity, forming the physical basis of heredity. See Heredity. ASEXIAL .NI) UxtlSVAL JloDES OF RePKODUC- TIO.N'. r.udding and its various forms are ex- amples of asexual reproduction. In certain in- sects and other animals the egg develops and gives rise to a mature individual without fer- tilization. (See Altebn.vtiox oe Gener.tions ; Pakthexogenesis. ) Hertwig calls this sexual reproduction with degenerated fertilization. Dif- ferent modes of reproduction (asexual, sexual, parthenogenesis, piedogenesis) may occur in the same species. The alternation of parthenogene- sis with jironounced sexual reproduction is called 'alternation of generations,' or heterogony. Consult: Hertwie. The Cell (New York, 1803) : id., Zoolofm (ib., 1902) ; Wilson, The Cell in Development and Inheritance (ib., 1897). REPRODUCTION (in plants). Although reproduction is a pliysiological phenomenon, it in animals; the morphological aspect, however, deals with structures peculiar to plants. Two types of ro])roduction are recognized among plants, namely, vegetative muUiplicalion and re- production by spurcx. { 1 ) Veuetative IMuLTlPLiCATiON. In this case the ordinary vegetative cells are used in repro- duction. (l"'igs. 1, 2.) In the simplest plants tlie adult body consists of a single vegetative cell, and when this cell divides two new ^9^^^^^ Fig. 1. VEGETATIVE KEPROnnCTION. 1. No.'itof. tilameTit breaking into colonies ; 2, Oscillatoria filament breaking up; 3, (Jloeocapsa forming new individ- uals by diviiliugri, Calothrix dividing by false branches. Flu. VEGETATIVE nEPBODUCTION IN A SAXIFRAGE. Stolons are put forth, upon which there arise buds, ca- pable of developing into plants like the parent. is also a prominent morphological phenomenon in so far as it involves important structures. The physiological aspects are essentiallj' the same as Fig. 3. ASEXUAL REPltonUCTION. 1. Tip of a green alga, showing swimming spores formed in ordinar.v cells and discharged ; 2, brancli of a red alga, containing tetraspores. plants are produced. The same power of cell division persists in all plants, but in the complex (mau.y-celled) plants much of it results, not in reproduction, but in the growth of the body. In such plants, however, the power of vegetative multiplication continues, as when Fig. 4. ASEXUAL nEPRonucTioN. 1, A fern spore germinating: "2. a. b. c, throe stages in germination from the spfiro of an alga; 3, spores (/>) ger- minating within a spore-case. new potato plants come from tubers (modified stems), or new grapevines from cuttings, or new begonias from pieces of a leaf, or when buds from one jibmt are grafted into the stem of an- other. In fact, vegetative multiplicntinn is very conspicuous in most plants, enabling them to spread with comparative raiiidity, ;is the -straw- beriy spreads by moans of its runners, or the couch grass by its rootstocks (underground stems ) .