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Show 22 INTRODUCTORY REl\1ARKS. CUAI'. I. therefore not exposed during succes ive y ar .t neh great vicissitudes of climate as are plants growlng out of doors. On some apparent and real Causes of Error in ·1~~!! Ex· periments.-It has been object~cl to su h xp rnn ·nt~ as 111ine that covering plants w1th a not, alth ugh onl ) fo'r a sh~rt time whilst in flower, may affect th ·ir h ttlth and fertility. I have seen no such eff ct . x · pt in one instance with a Myosotis, and the cov nng nuty n<~t. then have been the real canso of injury. But v n 1f the net were slightly injurious, ancl c rtai~ly it was not so in any high degree, as I could ju lg by th app ~tr ance of the plants and by comparing th ir f•rtility w1th that of neighbouring uncov red plant , it would not have vitiated my experiments; for in all th .~ 1nor iinportant cases the flowers were cross cl as w ll a · s Iffertilised under a net, so that they were tr at d in thi · respect exactly alike. As it is impossible to exclude such minut poll 'll-carrying insects as Thrips, flow rs which it wa · int IH1 d to fertilise with their own pollen may s 111 tin1 · hav been afterwards crossed with poll n brought by the 't' insects from another flower on the sa1n plant; but as we shall hereafter see, a cross of this kind do . not produce any effect, or at 1nost only a slight ne. vVhcn two or more plants were placed near one another under the same net, as was often clone, th r is smno real though not great danger of tho flow rs which were believed to be self-fertilised b ing aft 'rward crossed with pollen brought by Thrir s fron1. a distinct plant. I have said that tho dang r ]s not gre<tt, because I have often found that plants which are self-sterile, unless aided by insects, remained sterile when several plants of the same species were placed CHAP. I. INTRODUCTORY REMARKS. 23 under the san1e net. If, however, the flowers which had been presumably self-fertilised by me were in any case after.w~rds crossed by Thrips with pollen brought from . a distinct plant, crossed seedlings would have been Included amongst the self-£ertilised · but it should be especially observed that this occu1Te~ce would tend to diminis~ and n~t. to increase any superiority in average height, fertility, &c., of the crossed over the self-fertilised plants. As the flowers which were crossed were never castrated, it is probable or even almost certain that I sometimes failed to cross-fertilise them effectually, and ~ha.t they were afterwards spontaneously self-fertilised. Th1s would have been most likely to occur with dichogamo~ s species, for without much care it is not easy to p_e~·ceive whether their stigmas are ready to be fertilised when the anthers open. But in all cases, as the flo":ers were protected from wind, rain, and the a~cess ?f Insects, any pollen placed by me on the stigmatic surfa?e whilst it was immature, would generally have remained there until the stigma was mature· ~nd the flowers would then have been crossed as wa~ 1ntende~.. Nevertheless, it is highly probable that self-!ertihsed seedlings have sometimes by this means got Included amongst the crossed seedlings. The effect would ~e,. a.s in the former case, not to exaggerate but to diminish any average superiority of the crossed over the self-fertilised plants. Errors arising from the two causes just named, and from others,-such. as some of the seeds not having bee~ th~roughly npened, though care was taken to avoid this error-the sickness or unperceived injury of a~y ?f the plants,-will have been to a large extent ehmina ~e?, 1n those cases in which many crossed and self-fertilised plants were measured and an averao·o 0 |