| Popis: |
In Solidago Purshii Porter, there are two cytodemes, one diploid (2n = 18) and the other tetraploid (2n = 36), which can be distinguished on the basis of the length of the guard cells of their leaf stomata. The diploid plants possess guard cells which are smaller than those of the tetraploid. These two entities of S. Purshii thus belong to the groups of cytodemes in which there exists a positive correlation between the chromosome numbers and the dimensions of certain quantitative morphological characters, such as the length of the guard cells.In groups in which the variation curves of the characters of the different cytodemes overlap little or not, the geographical distribution of the different cytodemes of the complex may be studied much more easily, rapidly, and comprehensively by measuring the length of the guard cells of herbarium specimens than by making chromosome number determinations. The data presented indicate that when sampling is made under the conditions described in this paper, determinations of the length of two guard cells of stomata of one mid-leaf per individual, made on two individuals per colony, is satisfactory for that purpose, on the average. However, it is recommended, as a safeguard, to increase these figures by a few units, and to make always a few direct chromosome number determinations to check the accuracy of this rapid technique. These direct determinations may also be necessary to know the chromosome numbers of certain extreme individuals of the different cytodemes which have guard cells that differ little in length. |
