1.
Yucca recurvifolia (
n=30) has five large bivalents and twenty-five small bivalents as has
Yucca flaccida (cf. O'MARA 1931). The chiasma frequencies of large chromosomes at the diplotene stage, and at the early and late diaphases are 4.34, 3.07 and 2.48 respectively, while those of small chromosomes at the diplotene stage and at the early diaphase are 2.06 and 1.33 respectively. Evidently, the chiasma frequency is not a direct function of the chromosome length, as in
Stenobothrus, Chorthippus,
Hyacinthus amethystinus, Scilla and
Urginea. The terminalisation is complete at the early diaphase in small chromosomes in
Yucca, but incomplete at the late diaphase in large chromosomes.
2.
Scilla peruviana (2
n=16) has one pair of long chromosomes, four pairs of medium chromosomes and three pairs of short chromosomes. These types of bivalent may be readily recognized during the diplotene and metaphase stages of meiosis, so this provides a favourable material for an analysis of the chromosome complement. The observations made at the prophase of
Scilla give results similar to those on the chiasma behaviour in
Yucca. One of the short bivalents was attached to the nucleolus and a satellited bivalent was found at diaphase.
3.
Urginea scilla 2
n=40(4
b) has eight large chromosomes and thirty-two small chromosomes. The results of the observations made on the prophase chromosomes of
Urginea resemble those found with
Yucca except as regards the formation of tetravalents in the former. Tetravalent configurations were traced at both the diplotene and diaphase stages. Terminalisation of chiasmata was ascertained in both large and small tetravalents. The chromosome configurations were clearly explained by the pairing blocks theory (cf. DARLINGTON and MATHER 1932; STONE and MATHER 1932).
4. The pairing of chromosomes before the split is conditioned by chromomeres which may pair by their individual homology and after the split it is conditioned by chiasmata, and the chiasma frequency and distribution are influenced by genetical genes.
5. The reduction in the number of chiasmata may be chiefly due to terminalisation, for apparent chiasmata open out between the pachytene and diplotene stages and ‘break and join’ of crossed chromatids at a chiasma (SAX 1930, 1932 a) does not occur.
6. The position of the spindle fiber attachment, chromosome length and interference were chiefly discussed with reference to chiasma behaviour. The symmetrical relation of the chromatids in bivalents with two chiasmata in
Paeonia, Larix, Tsuga and
Allium is explained on the BELLING'S chiasma theory (1933) with regard to interference.
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