404 



FOURTH GROUP. — SEED-PLANTS. 



instance in the erect orthotropous ovules of Polygonum noticed above, that the 

 micropyle hes ahnost immediately opposite to the lower extremity of the style, so that 

 the downward-growing pollen-tube must necessarily encounter it. In the Compositae 



(Fig. 310), where there is also a single but 

 anatropous ovule, the micropyle is directed 

 towards the bottom of the ovary. Here, as 

 in Senecio Doria, two strips of conducting 

 tissue are found in the ovary right and left 

 from the median line of the ovule ; these 

 strips are continuous with the conducting 

 tissue of the style, descend to the bottom of 

 the ovary and there unite beneath the 

 micropyle which is bounded by the funicle. 

 The cells of the funicle which lie next the 

 micropyle also assume the character of a 

 conducting tissue; and thus a thread of 

 mucilage stretches from the bottom of the 

 ovary to the micropyle, and the pollen-tube 

 has its way marked out for it from the 

 stigma to the micropyle by the conducting 

 tissue, in which the cell-walls are changed 

 into mucilage. In other cases the cells of 

 the placenta (or of the outer integument) 

 secrete a mucilage in which the pollen-tube 

 developes. Thus the stigma is always in 

 connection with the cavity of the ovary or 

 with the ovules, either by means of a spongy 

 tissue or of a canal, the walls of which 

 supply the secretion mentioned above, or 

 this secretion is the product of the trans- 

 formation of their membrane. Each loculus 

 of a plurilocular ovary is of course in communication with the style. The presence of 

 a cellular tissue explains the fact, that in the very great majority of cases pollinated 

 flowers are found to have been fertilised. 



As regards the arrangements connected with pollination in flowers', it has been 

 already pointed out that in hermaphrodite flowers the stigmas are not usually 

 dusted with pollen from the same, but from some other flower. But cleistogantous 

 flowers are systematically fertilised by their own pollen. These are small flowers which 

 never open, and are found on a number of plants, such as Viola and Lajniiiui amplexi- 

 caule, along with other and larger flowers which expand to the air and light ; in cleisto- 

 gamous flowers the conveyance of the pollen of one flower to the stigma of another 

 (cross-pollination) is necessarily excluded. The arrangements by which cross-polli7iation 

 and cross-fertilisatio7i are secured in ordinary expanding flowers are very various. A 

 few only can be briefly noticed here. In many flowers the relative positions of anthers 

 and stigmas render it impossible for the pollen of the one to reach the other. Other 

 flowers again are incapable of self-fertilisation, are self-sterile ; if the pollen from the 

 anthers of a flower reaches the stigma of the same flower, it either developes no tube, 



Fig. 33s. Development ol the inflorescence of Fictis 

 Carica. /<■ and //young^ inflorescence from without surrounded 

 by involucral leaves which in /// are at the base of the inflores- 

 cence; /o, //, and 1 1 In are longitudinal sections. The axis of 

 the inflorescence is depressed and becomes cup-shaped, while 

 a number of involucral leaves fjrow out of the mouth of the cup, 

 and the flowers from its inner surface. 



1 C. Sprengel, Das neuentdeckte Geheimniss d. Natur im Bau u. in d. Befruchtung d. Blumen, 

 Berlin, 1793. — Hildebrand, Geschlechtvertheilung bei d. Pflanzen, Leipzig, 1867. — H. Miiller, Die 

 Befruchtung d. Blumen durch Insekteu, Leipzig, 1873 ; — Id. Die Alpenblumen, ihre Befruchtung 

 durch Insekten u. ihre Anpassungen an dieselben, Leipzig, 1880. — Darwin, On the various con- 

 trivances by which Orchids are fertilised, London, 1862 ; — Id., The effects of cross- and self- fertilisation 

 in the Vegetable Kingdom, 1876. [See also reference in note on p. 390.] 



