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LIST OF ILLUSTRATIONS.

XV

FIG.

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67. Leaves of Horse Chestnut . 68. Ditto 69. Leaves of Acer 70. Leaves of Castanea 71. Maple Leaves on Chestnut 72. Ditto. 73. Black Poplar (P. nigru) 74. Acacia melanoxylon 75. Seedling of Acacia salicina 76. Eucalyptus (young) 77. Eucalyptus (old). 78. Pupa of Eunomia eagrus 79. Lamium and Urtica 80. Ranunculus aquatilis . 81. Bupleurum fructicosum . 82. Senecio vulgaris 83. Drosera rotundifolia 84. Drosera anglica 85. Plantago media 86. Plantago lanceolata . 87. Daphne 88. Lathyrus niger 89. Lathyrus aphaca . 90. Lathyrus nissolia 91. Furze (Ulex). Seedling 92. Cephalanılra palmata. Seedling 93. Hibiscus pedunculatus. Seedling 94. Passiflora cærulea. Seedling 95. Senecio campestris

113 114 116 119 I 20 I22 I 22 126 128 130 132 133 131 134 134 134 135 137 137 138 141 142 143 144 146

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THE flower of the common White Deadnettle (Lamium album, Fig. 1) consists of a narrow tube, somewhat expanded at the upper end (Fig. 2), where the lower lobe of the corolla forms a platform, on each side of which is a small projecting lobe (Fig. 3, m). The upper portion of the corolla is an arched hood (Fig. 3 co), under which lie four anthers (a a), in pairs, while between them, and projecting somewhat downwards, is the pointed pistil (st). At the lower part, the tube contains honey, and above the honey is a row of hairs almost closing the tube.

Now, why has the flower this peculiar form ?

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B

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THE DEADNETTLE.

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What regulates the length of the tube? What is the use of this arch? What lessons do these lobes teach us? What advantage is the honey to the flower? Of what use is the fringe of hairs? Why does the stigma project beyond the anthers ? and why is the corolla white, while the rest of the plant is green?

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Similar questions may of course be asked with reference to other flowers. Let us now see whether we can throw any light upon them.

At the close of the last century, Conrad Sprengel published a valuable book on flowers, in which he pointed out that the forms and colours, the scent, honey, and general structure of flowers, have reference to the visits of insects, which are of importance in transferring the pollen from the stamens to the pistil. This admirable work, however, did not attract the attention it deserved, and remained almost unknown until Mr. Darwin devoted himself to the subject. Our illustrious countryman was the first clearly to perceive that the essential service which insects perform to flowers consists not only in transferring the pollen

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CROSS-FERTILISATION.

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from the stamens to the pistil, but in transferring it from the stamens of one flower to the pistil of another. Sprengel had indeed observed in more than one instance that this was the case, but he did not altogether appreciate the importance of the fact.

Mr. Darwin, however, has not only made it clear from theoretical considerations, but has also proved it, in a variety of cases, by actual experiment. More recently Fritz Müller has even shown that in some cases pollen, if placed on the stigma of the same flower, has no more effect than so much inorganic dust; while, and this is perhaps even more extraordinary, in others, the pollen placed on the stigma of the same flower acted on it like a poison. This he observed in several species; the flowers faded and fell off, the pollen masses themselves, and the stigma in contact with them, shrivelled up, turned brown, and decayed; while flowers on the same bunch, which were left unfertilised, retained their freshness.

The importance of this “cross-fertilisation," as it may be called, in contradistinction to "self-fertilisation," was first conclusively proved by Mr. Darwin in his remarkable memoir on Primula (Linnean Journal, 1862), and he has since illustrated the same rule by researches on Orchids, Linum, Lythrum, and a variety of other plants. The new impulse thus given to the study of flowers has been followed up in this country by Hooker, Ogle, Bennett, and other naturalists, and on the Continent by Axell, Delpino, Hildebrand, Kerner, F. Müller, and especially by Dr. H. Müller, who has brought together the observations of others, and added to them an immense number of his own.

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