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Show 210 NICOTIANA TABACUM. C !IAP. VI. certain individuals are sexually incompatible, and wHl not produce offspring, although fertile with other individuals. But Kolreuter has recorded a case* which bears mora closely on our present one, as it shows that in the genus Nicotiana the varieties differ in their sexual affinities. He experimented on five varieties of the common tobacco, and proved that they wore varieties by showing that they were perfectly fertile when reciprocally crossed; but one of these varieties, if used either as the father or the mother, was more fertile than any of tho others when crossed with a widely distinct species, N . glnt ,;nosa. As the different varieties thus differ in their sexual affinitjes, there is nothing surprising in the individuals of the same variety differ:ing in a like manner to a slight degree. Taking the plants of the throe generations altogether, tho crossed show no superiority over the sclf-fcrtHised, and I can account for this fact only by supposing that with this species, which is perfectly self-fertile without insect aid, most of the individuals are in the same condition, as those of the same variety of the common pea and of a few other exotic plants, which have been self-fertilised for many generations. In such cases a cross between two individuals does no good; nor docs it in any case, unless the individuals differ in general constitution, either from so-called spontaneous variation, or from their progenitors having been subjected to different conditions. I believe that this is the true explanation in the present instance, because, as we shall immediately see, the offspring of plants, which did not profit at all by being crossed with a plant of the same stock, profited to an extraordinary degree by a cross with a slightly different sub-variety. The E.ffects of a Oross with a f r6sh S toclc.-I procured some seed of N. tabar;um from Kew and raised some plants, which formed a slightly different sub-variety from my former plants; as the flowers were a shade pinker, the leaves a little more pointed, and the plants not quite so tall. Therefore the advantage in height which the seedlings gained by this cross cannot be attributed to direct inheritance. Two of the plants of the third self-fcrtHised generation, growing in Pots II. and V. in Table LXXXVII., which exceeded in height their crossed opponents (as did their parents in a still higher degree) were fertili. ed with po~len from the Kew plants, that is, by a fresh stock. The seedlmgs "' 'Das Geschlecht der Pflanzen, Zweite F ortsotzung,' 1764, P· 55-GO. CHAP. VI. CROSS WITH A FRESH STOCK. 211 thus raised may be called the Kew-crossed. Some other flowers on the same two plants were fertilised with their own pollen, and the seedlings thus raised form the fourth self-fertilised generation. The crossed capsules produced by the plant in Pot II., Table LXXXVII., were plainly less fine than the selffertilised capsules on the same plant. In Pot V. the one fin~st capsule was also a self-fertilised one; but the seeds produced by the two crossed capsules together exceeded in number those produced by the two self-fertilised capsules on the same plant. Therefore as far as the flowers on the parent-plants are concerned, a cross with pollen from a fresh stock did little or no good; and I did not expect that the offspring would have received any benefit, but in this I was completely mistaken. The crossed and self-fertilised seeds from the two plants were placed on bare sanrl, and very many of the crossed seeds of both set~ germinated before the self-fertilised seeds, and protruded their radicles at a quicker rate. Hence many of the crossed seeds had to be rejected, before pairs in an equal state of germination were obtained for planting on the opposite sides of sixteen large pots. The two series of seedlings raised from the parentplants in the two Pots II. and V. were kept separate, and when fully grown were measured to the tips of their highest leaves, as shown in the following double table. But as there was no uniform difference in height between the crossed and self-fertilised seedlings raised from the two plants, their heights have been added together in calculating the averages. I should state that by the ?ccidental fall of a large bush in the greenhouse, several pla-nts m both the series were much injured. These were at once measured together with their opponents and afterwards throwri away. The others were left to grow to their full height, and were measured when in flower. This accident accounts for the small height of some of tho pairs ; but as all the pairs, whether only partly or fully grown, were measured at the same time, the measurements are fair. The average height of the twenty-~ix crossed plants in the sixt~ en pots of the two series is 63 · 29, and that of the twentySIX self-fertilised plants is 41 · 67 inches; or as 100 to 66. Tho supe~iority of the crossed plants was shown in another way, for In every one of the sixteen pots a crossed plant flowered before a self-fertilised one, with the exception of Pot VI. of tho s~cond series, in which the plants on the two sides flowered Simultaneously. p 2 |