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Show 238 SUMMARY OF MEASUREMENTS. CHAP. VII. CHAPTER VII. SUMMARY OF THE HEIGHTS AND WEIGHTS 01!' TliE CROSSED AND SELF·FEB'l'ILISED PLAN'£8. Number of species and pbnts measured-Tables given-rrcliminary remarks on tho oflspring of plnnts crossed by a fresh stock-Thirteen cases specially consi(lereri-'rho effects of crossing- a sclf-fertili, ed plant either by another self-fertilised plant or by an intercross cl plant of the old stock-Summary of tho results-Preliminary remarks on the crossed and self-fertilised plants of the same stock - The twentysix exceptional cases considered, in which the eros ed plants did not exceed greatly in height the self-fertilised-Most of these cases shown not to he real exceptions to the rule that cross-fcrtili8ation is beneficial-Summary of results-Relative weight;:; of tho crossed and self-fertilised plantr:;, THE details which have been given under the head of each species are so numerous and so intricate, that it is necessary to tabulate the results. In Table A, the number of plants of each kind which vYore raised from a cross between two individuals of the same stock: and from self-fertilised seeds, together with their mean or average heights, are given. In tho right-hand column, the mean height of tho crossed to that of the self-fertilised plants, the former being taken as 100, is shown. To make this clear, it may be advisable to give an example. In tho first generation of Ipomooa, six plants derived from a cross between two plants were measurerl, and. their mean height is 86 · 00 inches; six plants derived from flowers on the same parent-plant fertilised with their own pollen were measured, and their mean height is 65 · 66 inches. CHAP. VIL SUMMARY OF MEASUREMENTS. 239 From this it follows, as shown in the right-hand colu1nn, that if the mean height of the crossed plants be taken as 100, that of the self-fertilised plants is 76. The same plan is followed with all the other species. The crossed. and self-fertilised. plants wore generally grown in pots in competition with one another, and always under as closely similar conditions as could be attained. They were, however, sometimes grown in separate rows in th~ open ground. With several of the species, the crossed plants were again crossed, and the self-fertilised plants again self-fertilised, and thus successive generations were raised and n1easured, as may be seen in rrable A. Owing to this manner of proceeding, the crossed plants became in the later generations 1nore or less closely inter-related. In Table B the relative weights of the crossed and self-fertilised plants, after they had flowered and had been cut down, are given in the few cases in which they were ascertained. The results are, I think, more striking and of greater value as evidence of constitutional vigour than those deduced from the relative heights of the plants. The most important table is that of C, as it includes the relative hejghts, weights, and fertility of plants l'aised from parents crossed by a fresh stock (that is, by non-related plants grown under different conditions), or by a distinct sub-variety, in comparison with selffertilised plants, or in a few cases with plants of the same old stock intercrossed during several generations. The relative fertility of the plants in this and the other tables will be 1nore fully considered in a future chapter. |