. Botany of the living plant. Botany; Plants. HEREDITY AND VARIATION The essential features of meiosis arc ilh. I in 1 the case of mitosis it is convenient to distinguish a nu: very complicated proo Heterotype I m : Proj > a In the first stage {leptotene) (Fig. 430, a, 431, 1) the chroma as long slender threads with a general resemblance t the chn in the first stage of mitosis, except that the threads are single instead double. ...... ⢠* . â " â * \ - ⢠⢠⢠⢠1 ⢠/ i .: 77r-f H£ y ..; ' ' ' yf ⢠*. 1 .». Fig. 431. Meiosisâearly stages in Trillium erection. 1, leptotene ;
. Botany of the living plant. Botany; Plants. HEREDITY AND VARIATION The essential features of meiosis arc ilh. I in 1 the case of mitosis it is convenient to distinguish a nu: very complicated proo Heterotype I m : Proj > a In the first stage {leptotene) (Fig. 430, a, 431, 1) the chroma as long slender threads with a general resemblance t the chn in the first stage of mitosis, except that the threads are single instead double. ...... ⢠* . â " â * \ - ⢠⢠⢠⢠1 ⢠/ i .: 77r-f H£ y ..; ' ' ' yf ⢠*. 1 .». Fig. 431. Meiosisâearly stages in Trillium erection. 1, leptotene ; 2, EygOteoe ; r ra tene. (From Sansome, after Huskins and Smith, by courtesy of Scientific Horti- culture.) At the second stage {zygotene) (Fig. 430, b, 431, 2) the chromosomes become associated in pairs. This association is not a random one ; the chromosomes that pair are the corresponding, similarly constituted or homologous ones, one of each pair being derived from the male gamete, and the other from the female gamete, which initiate this sporophyte generation. In the third stage {pachytene, Fig. 431, 3) the paired chromosomes api shorter and thicker as a result of twisting round one another. At the fourth stage {diplotene, Fig. 430, c, d) the chromosomes split length- wise into halves so that each pair now consists of four intertwined threads (chromatids), held together in pairs by the centromeres, which do not split. At this stage, or immediately prior thereto, the members of a homologous pair interchange some of their parts, a process known as crossing-over, only two out of the four chromatids breaking and rejoining at any one point. The homologous chromosomes then begin to separate, but not completely, being still held together at certain pointsâchiasmataâas a result of crossing-over (Fig. 433). These four stages collectively constitute pt ; \ Metal During the fifth and sixth stage (diakinesis and metaphass) the four chroma tids continue to shorten
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Keywords: ., bookcentury1900, bookpublis, booksubjectbotany, booksubjectplants
