You are not currently logged in.
Access JSTOR through your library or other institution:
If You Use a Screen ReaderThis content is available through Read Online (Free) program, which relies on page scans. Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
The Cytology of Veltheimia viridifolia, Jacq.
L. C. Coleman
American Journal of Botany
Vol. 27, No. 10 (Dec., 1940), pp. 887-895
Published by: Botanical Society of America, Inc.
Stable URL: http://www.jstor.org/stable/2436557
Page Count: 9
You can always find the topics here!Topics: Chromosomes, Chromocenters, Gyres, Condensation, Anaphase, Chiasmata, Metaphase, Pollen, Prophase, Centromeres
Were these topics helpful?See somethings inaccurate? Let us know!
Select the topics that are inaccurate.
Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Preview not available
The number and morphology of the somatic chromosomes, as studied in pollen grain mitoses, were found to agree with the previous report of Taylor. A chromosome of the smallest category is regularly associated terminally with the single nucleolus of the pollen grain. No satellite could be found associated with this chromosome. The premeiotic nucleus contains both chromocenters and chromonematic filaments. The number of the former is less than the diploid number of chromosomes (40) and appears to be between 20 and 30. However, it is difficult to distinguish the smaller chromocenters from overlapping points of the chromonemata (the so-called nodes of the reticulum). The chromocenters have been found associated only with the shorter chromosomes possessing median constrictions. In the zygophase the pairing of homologues bearing chromocenters appears to start at the distal ends and to proceed towards the region of the primary constriction which pairs later. The chromocenters are usually still present at this stage and show up as paired more or less spherical bodies one-half of each on either side of the primary constriction. They take an intense Feulgen stain and are interpreted as portions of the chromonemata which have remained tightly coiled. Partial uncoiling has frequently been seen when the chromocenters appear as linear thickened bodies on the chromonemata. The manner of pairing and the persistence of the chromocenters into the pachyphase are considered as related to a great reduction of chiasma frequency in the neighborhood of the primary constriction. In the zygophase and more prominently in the pachyphase there is evidence of a differential condensation of the bivalents in the region of the primary constriction. This differential condensation is confined to the short bivalents and is invariably found in bivalents carrying chromocenters. This phenomenon is similar to that described by Darlington for Agapanthus and Kniphofia. However, in these latter forms, all the bivalents appear to show the phenomenon, whereas in Veltheimia this is not the case. The presence of a Feulgen positive centromere in the primary constriction thread is established. The centromere is relatively smaller than that figured by Darlington for Agapanthus and Kniphofia. In early diplophase, differences in condensation are shown in the opening bivalents. This is associated with a difference in chiasma distribution. The three or four longest bivalents show very much less condensation than do the shorter ones which, moreover, show the greatest thickness in the region of the primary constriction. In the long bivalents the chiasmata are somewhat uniformly distributed. In the shorter bivalents the chiasmata (2 or 1 in number) are usually situated towards the ends of the bivalents and away from the primary constriction. It is suggested that this differential condensation is generally associated with restricted crossing over in the region of the primary constriction and that the low crossing over in the differential scgments of ring-forming Oenotheras may be associated with similar phenomena. The chromonematic coiling at first and second metaphase and anaphase has been resolved for all the bivalents, even the smallest. Throughout, it shows the one common type of the simple coil. No evidence of double interlocking coils was present in the second anaphase as has been so frequently reported for other forms. The gyres at first and second anaphase, while similar in form, are unequal in number, the second anaphase showing about double the number of gyres of the first anaphase. That the coils of the second division correspond to minor coils in the first division seems out of the question. There is, at times, a change of direction of the gyres at first metaphase on opposite sides of a chiasma but this is not constant. The gyres at the anaphases are too fine to establish with certainty changes of direction if present. Meiosis proceeds with great regularity. No cases of lagging were found and only one bridge was observed in the large number of pollen mother cells examined.
American Journal of Botany © 1940 Botanical Society of America, Inc.