For ordinary operations, no especial outfit is necessary for the crossing of plants, but those who experiment largely will find that the work will be greatly facilitated by the use of a portable box in which the various requisites can be carried. If this box has a compartment for every article, the operator will see at a glance if anything is lacking before he goes to the field. Figs. [105] and [106] illustrate a convenient pollinating kit. This is made about 12 inches long, 9 inches wide and 3 inches deep. In the central portion is a compartment for bags and one for labels. At the right end, running crosswise, is a narrow compartment for string, and at its upper end is a small bottle of alcohol. Into this alcohol the scalpel and other tools are dipped whenever another kind of pollen is to be used, in order to destroy whatever pollen grains may adhere to them. In front is a compartment for a magnifying glass, and a long one to hold scalpel, brushes, crochet-hook and pencil. The note-book is held in the cover by a wire clasp.
Fig. 104. Treated squash flower.
It is rare that all the flowers which one pollinates will mature fruit. If one-half are successful on the average, the operator may be satisfied. External conditions have much to do with the success of the operation. Some species do not set well during very dry weather and some are impatient of confinement. But probably all plants which thrive under glass are more sure to give good results if pollinated under confinement, because conditions are under control.
Crossing of Flowerless Plants.—Ferns, lycopodiums, and their allies, pass through two stages of development, and fertilization takes place only in the first stage. When spores germinate, a small, thin, green tissue spreads over the soil. This tissue is the prothallus (or prothallium). Upon the prothallus, or somewhat sunken in it, the sexual organs appear. They are minute aggregations of cells. Some of these aggregations develop into sperm or male organs and some into germ or female organs. The sperm organ is known as an antheridium and the germ organ as an archegonium. Spermatozoids are formed in the antheridium, and these enter the archegonium and fertilize the germ cell. This fertilized germ cell then develops into the second stage of the species, or into that part which we know as the fern or the lycopod. During this second stage, the plant bears leaf-like organs and it also produces numerous spores. These spores will produce the prothallus again when sown. Ferns, therefore, are fertilized but once during their lifetime, and the spores are not the direct result of fertilization as are the seeds of flowering plants.
Fig. 105. Pollinating Kit, closed.
If ferns and other flowerless plants are to be crossed, therefore, the operation must be performed in the prothallic stage. It was long a matter of doubt among botanists as to whether crossing is possible among these plants, but it is now known that it does occur. It has been brought about repeatedly in cultivation. The sperm bodies are not transferred by hand, but the spores of the species between which crosses are desired are sown together and the transfer is allowed to take place naturally. The prothallia of ferns are nearly always diœcious (sexes borne on different plants), so that crossing in such cases is not improbable. Many spores should be sown to increase the chances of success, and care should be taken that the different kinds germinate simultaneously. Some species germinate quicker than others, and the operator must determine by previous trial what these differences are. (For methods of sowing spores, see [page 24].) Only a small part of the plants will be likely to be crosses. In one of Lowe’s experiments, only five plants out of 1,000 were undoubted crosses.