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Show 274 SUMMARY OF MEASUHEMENTS. CnAP. VII. cases in which the crossed plants were not taller than the self-fertilised plants in any 1narked degree. (1.) Dianthns caryophyll~ts (third genr>ratio11 ).-This plant was experimented on during four generations, in three of which the crossed plants exceeded in height the self-fertili sed generally by much more than five per cent.; and we have seen under Table C that the offspring from the plants of tho third self-fertilised generation crossed by a fresh stock profited in height and fertility to an extraordinary degree. But in this third generation the crossed plarits of the same stock were in height to the selffertilised only as 100 to 99, that is, they were practically equal. Nevertheless, when the eight crossed and eight self-fertilised plants wer.e cut down and weighed, the former were to the latter jn weight as 100 to 49! r:l1here can therefore be not the least doubt that the crossed plants of thiR species are greatly superior in vigour and luxuriance to the self-fertilised; and what was the cause of the self-fertilised plants· of the third generation, though so light and thin, growing up so as almost to equal the crossed in height, I cannot explain. (2.) Lobelia fulg ens (fiTst gene1·ation).-The crossed J>lants of this generation were much inferior in height to tho self-fertilised, in the proportion of 100 to 127. Althongh only two pairr; were 1neasured, which is obviously much too few to he trusted, yet from other evidence given under the head of this specie~, it is certain that the self-fertilised plants were very much more vigorous than the crossed. As I used pollen of unequal maturity for crossing and self-fertiU sing the parent-plants, it is possible that tho great difference in the growth of their offspring may have been due to this cause. In the next generation this source of error was avojdcd,· and many more plants were raised, and now the. average height of the twenty-throe crossed plants was to that of the twenty-three self-fertilised pl ants as 100 to 91. vVe can therefore hardly doubt that a cro. ~ is beneficial to this species. (3.) Petun£n vi o7a cea (third generation).- Eight crossed plants were to eight self-fertilised of the third generation in average height as 100 to 131; and at an early ago the crossed were inferior even in a st.ill higher degree. But it is a remarkable fact that in one pot in which plants of both lots grew extremely crowded, the crossed were thrice as tall as the self-fertilised. As in the two preceding and two succeeding generations, as well as CHAP. vn. TABLE A. 275 with plants raised by a cross with a fresh stock th d g e tl d d th 1 . . , e crosse r a y excee e e se f-fertilised in height, weight, and fertilit (when these two latter points were attended to) th t y t b , e presen cas ~J mThu s e looked at as an anomalv.; not affectincor t·h e g.e nera1 ru1 o . e most ~r?bable explanation is that the seeds from which the self-fertilise~ plants of the third generation were r~isod w~re not .well n~e~ed·;· for I have observed an analogous case With Ibens. Self-fertilised seedlings of this latter plant h. l were known to have been produced from seeds not well m'a~r~~1 gre:w from the first much more quickly than the crossed plants' which were raised from better matured seeds; so that bavin ' thu~ once .got a great start they were enabled ever afterwards t~ retmn their advantage. Some of these same seeds of the Ib ,· were sown on th e opposi.t .e . ens ~Ides of pots filled with burnt earth and pure sand, not contammg any organic matter. and now tl young .crossed seedlings grow during their short 'life to doub;: t~e height of the self-fertilised, in the same manner as occurred With the a hove two sets of seedlings of Petunia which were much crowded and th~s exposed to very unfavourable conditions. We hav~. seen also. 1n th~ eighth generation of Ipomma that sclffertihsed seedlmgs raised from unhealthy parent~ o-rew at first very ~uch more quickly than the crossed seedlings~ so that th~y wore for a long time much taller, though ultimately beaten by them. (4, 5, · 6.). Escl~sclwltda californica.- Four sets of measurements are given ~n ~~hle ~· In one of these the crossed plants exceed the self-fert1h ed In average height, ~o that this is not one of the exceptions here to be cons}dered. In two other cases the cross8.d equalled the self-fertilised in height within five per cent. ; and In the fourth case the self-fertilised exceeded the crossed by above this limit. We have seen in Table 0 that the ':hole advantage of a cross by a fresh stock is confined to fertihty, and so ~twas with the intercrossed plants of the same stock compared With the self-fertilised, for the former were in fertility to the latter as 100 to 89. The intercrossed plants thus have at least one important advantage over the self-fert.ilised. Moreover, ~he flowe~s ~n.the parent-plants when fertilised with pollen from <tnother Individual of the same stock yield far more seeds than wh.en self-fertiliRed; the flowers in this latter case being often (lUite sterile. vVe may therefore conclude that a cross does somA good, though it does not give to the crossed seedlings increased powers of growth. T 2 |