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. [Illustration: Fig. 106. Pollinating Kit, open.] ORDINAL INDEX. An Index to the natural orders or families mentioned in the Nursery List. A number in parentheses indicates the number of references to the family upon a page, if more than one occurs. Acanthaceæ, 99, 115, 125 (2), 130, 133, 149, 158, 170, 183, 188, 198, 210, 216, 264, 273, 277 (2) Alangieæ, 106 Alismaceæ, 101, 107, 144, 217 Amarantaceæ, 108, 109, 156, 207 Amaryllideæ, 105, 108, 109, 138, 140, 142, 147, 148, 164, 169, 171, 173, 179, 185 (2), 190, 192, 199, 203, 206 (2), 207, 208, 215, 229, 231, 240, 247, 254, 280, 285 Ampelideæ (Vitaceæ), 110, 162, 195 Anacardiaceæ, 111, 166, 180, 222, 251, 262, 266, 271 Anonaceæ, 113, 122, 123 Apocynaceæ, 102, 104, 107, 108, 110, 115, 132, 152, 153, 178, 222, 231, 254, 275, 278, 282, 284 Araliaceæ, 120, 172, 192, 201, 238, 240, 267 Aristolochiaceæ, 121, 123 Aroideæ, Araceæ, 100, 107, 109, 110, 111, 114, 121, 123, 146, 199, 226, 238, 240, 248, 261, 262, 266 (2), 270, 272, 275, 281 Asclepiadeæ, 111, 123, 124, 140, 149, 157, 194, 199, 205, 223, 246, 249, 271, 272 Begoniaceæ, 132 Berberideæ, 106, 130, 134, 155, 182, 203, 209, 213, 215, 254 Bignoniaceæ, 102, 110 (2), 135, 154, 160, 180, 208, 276 Bixineæ, 128, 136, 206 Boragineæ, 111, 122, 138, 167, 172, 181 (2), 201, 218, 225, 228, 234 (2), 275 Bromeliaceæ, 99, 135, 141, 180, 250, 278 Bruniaceæ, 126, 134, 142 Burseraceæ, 110, 129, 139, 144