'Das Geschlecht der Pflanzen, Zweite Fortsetzung' 1764 pages 55-60.) He experimented on five varieties of the common tobacco, and proved that they were 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 the others when crossed with a widely distinct species, N. glutinosa. As the different varieties thus differ in their sexual affinities, there is nothing surprising in the individuals of the same variety differing in a like manner to a slight degree.
Taking the plants of the three generations altogether, the crossed show no superiority over the self-fertilised, 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 does 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 EFFECTS OF A CROSS WITH A FRESH STOCK.
I procured some seed of N. tabacum 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-fertilised generation, growing in Pots 2 and 5 in Table 6/87, which exceeded in height their crossed opponents (as did their parents in a still higher degree) were fertilised with pollen from the Kew plants, that is, by a fresh stock. The seedlings 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 from the fourth self-fertilised generation. The crossed capsules produced by the plant in Pot 2, Table 6/87, were plainly less fine than the self-fertilised capsules on the same plant. In Pot 5 the one finest 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 sand, and very many of the crossed seeds of both sets 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 parent-plants in the two Pots 2 and 5 were kept separate, and when fully grown were measured to the tips of their highest leaves, as shown in Table 6/88. 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 accidental fall of a large bush in the greenhouse, several plants in both the series were much injured. These were at once measured together with their opponents and afterwards thrown away.