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Show 278 SUMMARY OF MEASUREMENTS. CHAP. VII. insects and a few self-fertilised plants were grown on the opposite sides of four pots. They attained to a nearly equal height, the crossed having a very sUgbt advantage. (18.) Vandellia nummular~folia.-Twonty cro sed plants mi 'ed from tho seeds of perfect flowers were to twenty self-fertilised plants, likewise raised from the seeds of perfect flowers, in height as 100 to 99. The experiment was repeated, with the sole difference that the plants were allowed to grow more crowded· and now the twenty-four tallest of the crossed plants were t~ tho twenty-four tallest self-fertilised plants in height as 100 to D4, and in weight as 100 to 97. Moreover, a laro-er number of the cross~d . than of the self-fertiJjsed plants grow to a moderate height. Tbe above-mentioned twenty crossP-d plants were also grown in competition with twenty self-fertilised plants raised from tl Le closed or cleistogene flowers, and their heights were as 100 to 94. Therefore had. it not been for the first trjal, in which the crossed plants were to the self-fertilised in height only as 100 to 99, this species might have been classed with 1 those in which the crossed plants exceed the self-fertilised by above five p~r cent. On the other hand, the crossed plants in tlw second tnal bore fewer capsules, and these contained fewer seeds, than did the self-fertilised plants, all the capsules having been produced by cleistogene flowers. The whole case therefore must be left doubtful. (19.) Pisum sativum (common pea).-Four plants derived from a cross between individuals of the same variety were in height to four self-fertilised plants belonging to the same variety as 100 to 115. Although this cross did no good, we have seen under Table C that a cross between distinct varieties adds O'reatJy to the height and vigour of the offspring; and it was there ~xplained that the fact of a cross between the individuals of the same variety not being beneficial, is almost certainly due to their having been self-fertilised for many generations, and in each generation grown under nearly similar conditions. (20, 21, 22.) Cunna warscewiczi.-Plants belonging to three generations were observed, and in all of three the crossed were approximately equal to the self-fertilised· the average height of the thirty-four crossed plants being to th~t of the same number of self-fertilised plants as 100 to 101. Therefore the crossed plants had no advantage over tho self-fertilised; and it is probab. le that the same explanation here holds good as in the case of P'isum sativum; for the flowers of this Canna are perfectly CHAP. VII. TABLE ·A .. . 279 self-fertile, and were never seen to be visited by insects in tho hothouse, so as to be crossed by them. This plant, moreover, has been cultivated under glass for several generations in pots, and therefore under nearly uniform conditions. The capsules produced by the crof;:s-fertilised flowers on the above thirty-four crossed plants contained more seeds than did the capsules pi'oduced by the self-fertilised fl0wers on tho self-fertilised plants, in the proportion of 100 to 85; so that in this respect crossing was bene:ficial. (23.) Prirnula sz'nensis.-The offspring of plants, some of which were .legitimately and others illegithnately fertilised with pollen from a distinct plant, were almost exactly of the same height as the offspring of self-fertilised plants; but the former with rare exceptions flowered before the latter. I have shown ~n my paper on dimorphic plants that this species is commmdy raised in England from self-fertilised seed, and the plants from having been cultivated in pots have been subject~d to nearly uniform conditions. Moreover, many of them are now varying and changing their charactel', so as to . b~come in a greater or less degree equal-st.yled, and in consequence highly self-fertile. Therefore I believe that the cause of the m·ossed plants not exceeding in height the self-fertilised is tho same as in the two previous cases of Pisum srdivum and Canna. (24, 25, 26.) N1:cotia:na tahacum.-Four sets of measurements were made; in one, the self-fertilised plants greatly exceeded in height the crossed, in two others they wore approximately equal to the crossed and in the fourth were beaten by them; but this latter case d;es not here concern us. The individual plants differ in constitution, so that the descendants of some profit by their parents having been intercrossed, whils.t others do not. Taking a:ll three generations together, the twenty-seven crossed plants were in hejght to the twenty-seven self-fertiUsed plants as 100 to 96. This excess of height in the crossed plants, is so small compared with that displayed by the offsp1·ing from the same mother-plants when crossed ~y a slightly different variety, that we may suspect (as explained under Table C) that.most of the individuals belonging to the variety which served as the mother-plants in my experiments, had acquired a nearly similar constitution, so as not to profit by being mutually intercrossed. Reviewina these twenty-six cases, in which the crossed pla~ts either do not cxce d the self-fertilised |
