The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 37

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 14 4/8 : 27 6/8.

Pot 1 : 78 4/8 : 8 6/8.

Pot 1 : 9 : 56.

Pot 2 : 60 4/8 : 16 6/8.

Pot 2 : 44 6/8 : 7.

Pot 2 : 10 : 50 4/8.

Pot 3 : 57 1/8 : 87 (A).

Pot 3 : 1 2/8 : 81 2/8 (B).

Pot 4 : 6 6/8 : 19.

Pot 4 : 31 : 43 2/8.

Pot 4 : 69 4/8 : 4.

Pot 5 : 99 4/8 : 9 4/8.

Pot 5 : 29 2/8 : 3.

Total : 511.63 : 413.75.

The thirteen crossed plants here average 39.35, and the thirteen self-fertilised plants 31.82 inches in height; or as 100 to 81. But it would be a very much fairer plan to exclude all the starved plants of only 10 inches and under in height; and in this case the nine remaining crossed plants average 53.84, and the seven remaining self-fertilised plants 51.78 inches in height, or as 100 to 96; and this difference is so small that the crossed and self-fertilised plants may be considered as of equal heights.

In addition to these plants, three crossed plants were planted separately in three large pots, and three self-fertilised plants in three other large pots, so that they were not exposed to any compet.i.tion; and now the self-fertilised plants exceeded the crossed in height by a little, for the three crossed averaged 55.91, and the three self-fertilised 59.16 inches; or as 100 to 106.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

TABLE 6/86. Nicotiana tabac.u.m (third generation). Seedlings from the self-fertilised plant A in pot 3, Table 6/85, of the last or second generation.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: From Self-fertilised Plant, crossed by a Crossed Plant.

Column 3: From Self-fertilised Plant again self-fertilised, forming the third Self-fertilised generation.

Pot 1 : 100 2/8 : 98.

Pot 1 : 91 : 79.

Pot 2 : 110 2/8 : 59 1/8.

Pot 2 : 100 4/8 : 66 6/8.

Pot 3 : 104 : 79 6/8.

Pot 4 : 84 2/8 : 110 4/8.

Pot 4 : 76 4/8 : 64 1/8.

Total : 666.75 : 557.25.

As I wished to ascertain, firstly, whether those self-fertilised plants of the last generation, which greatly exceeded in height their crossed opponents, would transmit the same tendency to their offspring, and secondly, whether they possessed the same s.e.xual const.i.tution, I selected for experiment the two self-fertilised plants marked A and B in Pot 3 in Table 6/85, as these two were of nearly equal height, and were greatly superior to their crossed opponents. Four flowers on each plant were fertilised with their own pollen, and four others on the same plants were crossed with pollen from one of the crossed plants growing in another pot. This plan differs from that before followed, in which seedlings from crossed plants again crossed, have been compared with seedlings from self-fertilised plants again self-fertilised. The seeds from the crossed and self-fertilised capsules of the above two plants were placed in separate watch-gla.s.ses and compared, but were not weighed; and in both cases those from the crossed capsules seemed to be rather less numerous than those from the self-fertilised capsules. These seeds were planted in the usual manner, and the heights of the crossed and self-fertilised seedlings, when fully grown, are given in Tables 6/86 and 6/87.

The seven crossed plants in the first of these two tables average 95.25, and the seven self-fertilised 79.6 inches in height; or as 100 to 83. In half the pots a crossed plant, and in the other half a self-fertilised plant flowered first.

We now come to the seedlings raised from the other parent-plant B.

TABLE 6/87. Nicotiana tabac.u.m (third generation). Seedlings from the self-fertilised plant B in pot 3, Table 6/85, of the last or second generation.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: From Self-fertilised Plant, crossed by a Crossed Plant.

Column 3: From Self-fertilised Plant again self-fertilised, forming the third Self-fertilised generation.

Pot 1 : 87 2/8 : 72 4/8.

Pot 1 : 49 : 14 2/8.

Pot 2 : 98 4/8 : 73.

Pot 2 : 0 : 110 4/8.

Pot 3 : 99 : 106 4/8.

Pot 3 : 15 2/8 : 73 6/8.

Pot 4 : 97 6/8 : 48 6/8.

Pot 5 : 48 6/8 : 81 2/8.

Pot 5 : 0 : 61 2/8.

Total : 495.50 : 641.75.

The seven crossed plants (for two of them died) here average 70.78 inches, and the nine self-fertilised plants 71.3 inches in height; or as 100 to barely 101. In four out of these five pots, a self-fertilised plant flowered before any one of the crossed plants. So that, differently from the last case, the self-fertilised plants are in some respects slightly superior to the crossed.

If we now consider the crossed and self-fertilised plants of the three generations, we find an extraordinary diversity in their relative heights. In the first generation, the crossed plants were inferior to the self-fertilised as 100 to 178; and the flowers on the original parent-plants which were crossed with pollen from a distinct plant yielded much fewer seeds than the self-fertilised flowers, in the proportion of 100 to 150. But it is a strange fact that the self-fertilised plants, which were subjected to very severe compet.i.tion with the crossed, had on two occasions no advantage over them. The inferiority of the crossed plants of this first generation cannot be attributed to the immaturity of the seeds, for I carefully examined them; nor to the seeds being diseased or in any way injured in some one capsule, for the contents of the ten crossed capsules were mingled together and a few taken by chance for sowing. In the second generation the crossed and self-fertilised plants were nearly equal in height. In the third generation, crossed and self-fertilised seeds were obtained from two plants of the previous generation, and the seedlings raised from them differed remarkably in const.i.tution; the crossed in the one case exceeded the self-fertilised in height in the ratio of 100 to 83, and in the other case were almost equal. This difference between the two lots, raised at the same time from two plants growing in the same pot, and treated in every respect alike, as well as the extraordinary superiority of the self-fertilised over the crossed plants in the first generation, considered together, make me believe that some individuals of the present species differ to a certain extent from others in their s.e.xual affinities (to use the term employed by Gartner), like closely allied species of the same genus. Consequently if two plants which thus differ are crossed, the seedlings suffer and are beaten by those from the self-fertilised flowers, in which the s.e.xual elements are of the same nature. It is known that with our domestic animals certain individuals are s.e.xually incompatible, and will not produce offspring, although fertile with other individuals. (6/3. I have given evidence on this head in my 'Variation of Animals and Plants under Domestication'

chapter 18 2nd edition volume 2 page 146.) But Kolreuter has recorded a case which bears more closely on our present one, as it shows that in the genus Nicotiana the varieties differ in their s.e.xual affinities.

(6/4. '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 s.e.xual 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 const.i.tution, 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. tabac.u.m 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.