410 FERTILIZATION AND FORMATION OF FRUIT IN PHANEROGAMS. 



which the pollen-tube penetrates show no demonstrable diflference from their neigh- 

 bours. In this respect the Grasses differ from the very large number of plants 

 which, although they do not possess an open style-canal as in Lilium Martagon, 

 have a loose axial string of mucilaginous tissue in their styles through which the 

 pollen-tubes readily penetrate. Examples of this condition are the Solanacese and 

 Scrophulariacese. In other cases the conducting tissue is not differentiated from its 

 surroundings, so that the whole of the substance of the style and stigma serves for 

 the conduction of the pollen-tubes, as in Cistus, Helianthemum, and Orchids. 



A curious condition prevailing in the Cactuses has been observed in the fre- 

 quently mentioned Cereus. Here, although a narrow style-canal is present, the 

 pollen-tubes prefer to make their way to the ovary embedded in the tissue surround- 

 ing the canal. From this it would appear that it is of advantage for the pollen- 

 tubes to travel thus inclosed by other tissues. 



Different again is the course followed by the pollen-tubes in the Malvaceae and 

 in many Caryophyllaceae. The stigmas here are in some degree like those of 

 Grasses. As there, so here, the superficial cells are produced into long, thin-walled 

 papillae; to these papillae the pollen-grains become attached by the agency of insects. 

 The pollen-tube as it develops from the grain at once perforates the wall of a 

 stigmatic papilla and continues its growth in the cell-cavity. The course now fol- 

 lowed is remarkable. In the Corn Cockle (Agrostemma Oithago) the pollen-tube 

 often zigzags from one side of the cavity of the stigmatic papilla to the other, not 

 infrequently taking first of all the wrong direction and bending up towards the tip 

 of the papilla, and then bending completely round again. Having reached the base 

 of the papilla, the tube bores through into the conducting tissue in the interior of 

 the style, but in its further course down to the ovary grows solely between the cells, 

 not in them. It sometimes happens that more than one tube arises from a single 

 pollen-grain; the accessory ones, however, are for purposes of firmer attachment, 

 and though they occasionally enter a stigmatic papilla do not continue their growth 

 down the tissue of the style. One functional pollen-tube only is produced from 

 each pollen-grain. In the Malvaceae (e.g. Malva sylvestris) the pollen-tube entirely 

 fills a stigmatic papilla, broadening out at the base. Ultimately the contents of the 

 tube escape from their membrane and travel down the style in an elongated mass, 

 destitute of wall, like the plasmodium of a Myxomycete. 



Whatever be the manner of its travelling, whether with or without a wall 

 of its own, the aim of the protoplasm of the pollen-grain is to reach one of the 

 ovules in the ovary. Having entered the cavity of the ovary, a pollen -tube 

 shapes a course for an ovule. The particular portion of the ovule aimed at — in 

 the vast majority of flowering plants — is the micropyle (cf. vol. i. p. 644), the 

 little receptive spot at which the coats of the ovule are discontinuous, and at 

 which access to the embryo-sac (wherein is contained the egg-cell) can be gained. 

 Only comparatively rarely is the micropyle situated immediately below the point 

 at which the pollen-tube must enter the ovary, as represented, for instance, in 

 fig. 208^, p. 74. Sometimes the micropyle is directed towards the side wall of 



