OCR Text |
Show 258 SUMMARY OF MEASUREMENTS. UIIAl'. VIr. seedlings \Yere grown in competition \vith one another, and difD red greatly in height and fertility. ~For the offspring from tho cross with a fresh ·tock cxcr \ded. in height the intercrossed plants in the ratio of 100 to 78; and this is nearly the same excess which tho jntercrossed had over the self-fertilised plants in all ten generations taken together, narn ly, as 100 to 77. ~rho ]!lants raised from the cross with a fresh stoek were also greatly superior in fertility to the intorerossod, na1nely, in the ratio of 100 to 51, as judged by the relative weight of the seed-capsules produced ,by an equal number of plants of the two sets, both havjno· been left to be naturally fertilised. It should be especially observed that none of tho plants of either lot were tho product of self-fertilisation. On the contrary, the intercross d plants had certainly boon crossed for the last ten gcn rations, and probably, during all previous generations, as we may infer from the structure of the flowers and from the frequency of the visits of huinl1lebeos. And so it will have been with the parent-plants of the fresh stock. The whole great difference in height and fertility between the two lots must be attributed to tho one being the product of a cross with pollen from a fresh stock, and the other of a cross ·between plants of the same old stock. This species offers another interesting case. In the five first generations in which intorcrossccl and selffertilised plants w re put into competition with one another, every single intcrcrossc<l plant boat its s~lffertilised antagonist, except in one instance, iu whiCh they were equal in height. But in tho sixth goneration a plant appeared, named by 1ne the H or~, .reInarkable for its tallness and incrct~sc<l self-fertility, and which transmitted its characters to the next three g nerations. The children of Hero were again self~ CHAP. VII. TABLE C. 259 fertilised, forming the eighth self-fertilised generation, and were likewise intercrossed one with another; but this cross between plants which had been subjected. to the same conditions and had been self-fertilised during the seven previous generations, did not effect the least good; for the intercrossed grandchildren wore actually shorter than the self-fertilised grandchildren, in the ratio of 100 to 107. We here see that the mere · act of crossing two distinct plants does not by itself benefit the offspring. This case is almost the converse of that in the last paragraph, in which the offspring profited so greatly by a cross with a fresh stock. A similar tri~l was made with the descendants of Hero in the following generation, and with the same result. But the trial cannot .be fully trusted, owing to the extremely unhealthy conuition of the plants. Subject to this same serious cause of doubt, even a cross with a fresh stock did not beD;e:fit the greatgl~ andchildren of Hero; and if this were really the case, it is the greatest anomaly observed by me in all my exporimen ts. (~.) Mimulus luteus.-During the three first generations the intercrossed plants taken together exceeded in height the self-fertilised taken together, in the ratio of 100 to 65, and in fertility in a still higher degree. In the fourth generation a new variety, which grew taller and had whiter and larger flowers than the old varieties, began to prevail, especially amongst the selffertilised plants. This variety transmitted its characters with remarkable fidelity, so that all the plants in the later self-fertilised generations belonged to it. These consequently exceeded the intcrcrossed 'plants considerably in height. 'l1hus in the seventh generation tho intercrossed plants were to the self-fertilised in height as 100 to 137. It is a more remarkable fact that the s 2 |