. An introduction to the structure and reproduction of plants. Plant anatomy; Plants. VARIATION 375 involve differences in the number of constituent parts, such as perianth-segments, leaflets in a compound leaf, etc. It is easiest to study variation if some character is selected which is capable of exact measurement, as, for instance, the length of the leaf in the Privet, the length of the Runner Bean seed, or the number of ray-florets in the Daisy. Taking the first of these cases as an example, it will be found that if, say, a thousand leaves are measured, the difference between the length of
. An introduction to the structure and reproduction of plants. Plant anatomy; Plants. VARIATION 375 involve differences in the number of constituent parts, such as perianth-segments, leaflets in a compound leaf, etc. It is easiest to study variation if some character is selected which is capable of exact measurement, as, for instance, the length of the leaf in the Privet, the length of the Runner Bean seed, or the number of ray-florets in the Daisy. Taking the first of these cases as an example, it will be found that if, say, a thousand leaves are measured, the difference between the length of the shortest and the longest is quite considerable, and that the majority of the leaves are of an average size. If all the thousand leaves be classified ac- cording to their lengths into separate groups, differing b)? increments of one millimetre, and the number of leaves in each group counted, those containing the smallest number will be found to be situated at the two extremes. Between these points the number of leaves of each particular length will be found to increase with considerable regularity, as the size of its individuals approaches that exhibited by the majority. This most frequent size is termed the mode, and often corresponds very closely to the arithmetical mean of all the measurements. By plotting a curve, in which the ordinates represent the number of individuals in each group, and the abscissa: tha re- spective lengths, the variation can be graphically represented. Fig. 2i8.—Three equal-aged individuals of the Marsh Cudweed (Gnaphalium iiliginostim) from the same locality, showing individual variation. All three are in fruit. (Natural size.). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Fritsch, Felix Eugene, 1879-; Salisbury, E. J. (Edward James), Sir, b. 1886. London, G. Bell and son
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