Search for a command to run...
1. The investigation of 9 species and varieties of Spinacia revealed that each of the types examined contained 2 relatively large pairs of somatic chromosomes with sub-median spindle fiber attachment constrictions, 2 medium sized pairs with sub-terminal attachment constrictions, and 2 relatively small pairs with subterminal constrictions. One of the small pairs is characterized by the presence of trabants an the shorter arm. There is thus no cytological evidence to suppart the view that Spinacia oleracea L. and the so-called S. tetandra Stev. are distinct species. An examination of the chromosome complements of male, female and intersexual plants failed to reveal any heteromorphic allosomes. The metaphase chromosomes of the dermatogen of Spinacia are characterized by an affinity between homologues and by widely divergent chromatids.2. The chromosome numbers of 4 species of the Chenopodiaceae, hitherto uninvestigated somatically, are as follows: Sueda linearis Moq., 2n=54; Kochia scoparia L., 2n=18; Salsola Kali L. var. tragus L., 2n=36; Chenopodium ambrosioides L., 2n=32. The chromosome situation in the Chenopodiaceae, so far as investigated, appears to be based an the number x=9, rather than x=3, although the former number may have been secondarily derived from the latter. A reduplication of an original 3 might conceivably account for the chromosome situation in Spinacia.3. Somatic doubling of the chromosome complement was found to occur in the periblem of the root tip in Kochicc scoparia and in each of the species and varieties of Spinacia investigated.4. The results of a statistical investigation reveal that: (a) There is an apparent decrease in the length of the prophase period relative to that of the metaphase period from the periphery to the center of the root tip in Spinacia. The prophase is relatively longest in the dermatogen, shorter in the periblem and shortest in the plerome. (b) In the region of somatic doubling in the periblem of Spinacia the duration of the prophase period relative to that of the metaphase period is apparently no longer in instances where the chromosomes are paired than where they are unpaired; and that if the prophases with paired chromosomes are actually of longer duration than the prophases with unpaired chromosomes in point of time, then the metaphases with paired chromsomes are longer than the metaphases with unpaired chromosomes.5. Certain observations which were out of harmony with the hypothesis held by previous investigators to explain somatic doubling in Spinacia have led to the development of a new hypothesis which takes into account these formerly anomalous observations. This hypothesis is based an the conception that chromosomal division and nuclear division are not always interdependent, but may be influenced by environmental factors so that they behave in an asynchronous manner. The division of the chromosomes and the separation of the division products occurring at a faster rate than the division of the nucleus as a whole, ultimately results in two chromosomal cleavages occurring within one nuclear cycle. When this occurs the chromosome number is doubled. On this hypothesis the freauency with which doubling occurs and the height to which the chromosome number mounts are dependent upon the rapidity of the development of the environmental factors which distort the normal synchronism of nuclear and chromosomal behavior. If this development begins in or very near the promeristem and progresses rapidly it may result in the formation of octosomatic cells before the natural cessation of meristematic activity in the oldest part of the root tip occurs.