[190]

Ipomoea nil 20th Ipomoea Jun 20-21. 1878.

21' same plant both days

Fig 23. A

(Nat size some lettering)

[Figure]

Temp 20° C. (dates)

Fig 23A (Circumnutation of Cotyledon, traced on vertical glass. illuminated during the day rather feebly from vertically above) Apex of Leaf dot 4.1 from the glass — Cotyledon with petiole 1.6 of inch in length— apex 4.1 from the vertical glass

[190v]

33 644

Chap. E 10

would most or all of the genera in the Asclepiadæ and Apocynaceæ; but the lists are confined to the results species which were of were actually experimented on. The results can be considered as only approximately accurate, for fertility is so variable a character, that many trials ought to be made each species ought to have been tried many times.

The above number of genera, namely 94, is as nothing to the host of living plants; but the mere fact that of half of them are being sterile within the specified degree when insects are excluded is a striking circumstance one; for it may be inferred that whenever pollen has to be carried from the anthers to the stigma or from flower to flower in order to ensure full fertility, there is at least a good chance of cross-fertilisation. I do not, however, believe that if all known plants were thus tried in the above manner half would be found to be sterile in the above degree; for many flowers were selected for experiment which presented some remarkable structure; and such flowers often require insect-aid. Thus out of the 47 genera in the first list of plants which were more or less sterile plants when insects

[Cross and self fertilisation, p. 370: "If the genera in this latter order, as well as in the Asclepiadae and Apocynaceae, had been included, the number of species which are sterile if insects are excluded would have been greatly increased; but the lists are confined to species which were actually experimented on. The results can be considered as only approximately accurate, for fertility is so variable a character, that each species ought to have been tried many times. The above number of species, namely, 125, is as nothing to the host of living plants; but the mere fact of more than half of them being sterile within the specified degree, when insects are excluded, is a striking one; for whenever pollen has to be carried from the anthers to the stigma in order to ensure full fertility, there is at least a good chance of cross-fertilisation. I do not, however, believe that if all known plants were tried in the same manner, half would be found to be sterile within the specified limits; for many flowers were selected for experiment which presented some remarkable structure; and such flowers often require insect-aid. Thus out of the forty-nine genera in the first list, about thirty-two have flowers which are asymmetrical or present some remarkable peculiarity; whilst in the second list, including species which are fully or moderately fertile when insects were excluded, only about twenty-one out of the forty-nine are asymmetrical or present any remarkable peculiarity."]