OVULES OF AKGIOSPERMIA. 633 



where the apex of the embryo-sac grows out from the micropyle to 

 meet the pollen-tubes, and in such plants as EpJiedra or Welwitschia, 

 wherein the coat of the ovule is prolonged into a styliform process. 

 In Santalum album and some other plants the embryo is developed 

 entirely outside the nucleus, in the protruded part of the sac. 

 Schacht says that the embryonic vesicles in Santalum, Crocus, and 

 a few other genera, are much elongated ; the lower end becomes 

 rounded off into a cell, while the other end projects beyond the 

 embryo-sac into a slender tubular prolongation into the micropyle. 

 The sides of this are striated and gave rise to the appellation j^/orm 

 appendage, which is considered by Strasburger to correspond with 

 the canal-cell of Cryptogams. It is not clear whether the protru- 

 sion just alluded to is really from the embryo-sac or from the 

 germinal vesicle. 



Passage of the Pollen-tubes. When the pollen-tubes are formed 

 in the stigma they gradually elongate by growth at the apex into 

 tubes which pass down the canal of the style when this exists, the 

 latter being sometimes several inches long. The time occupied in 

 this growth varies from a few hours to several weeks. In the 

 Hazel-nut and other similar plants the pollen falls on the stigma 

 in spring before the ovules are even formed. The pollen-tubes 

 derive their sustenance from the tissues through which they pass, 

 and mostly die away above as they grow below ; and the stigma 

 withers soon after the pollen-tubes have penetrated. 



It is remarkable that the stigma remains fresh for a considerable time 

 in unfertilized ovaries ; and in the occasional cases of development of an 

 unfertilized ovule, which has been observed in some dioecious plants, as 

 Ccelebogyne, Hemp, Mercurialis, &c., the stigma does not wither. 



Pollen-tubes. The pollen-tubes are exceedingly minute, the diameter 

 averaging from 4-^0-5- or TOFO f an i ncn But Amici estimated the 

 number of pollen-tubes formed from the pollen-masses of Orchis Morio 

 at 120,000. Experiments have shown, however, that, under favourable 

 circumstances, a very few pollen-grains suffice for even a many-ovuled 

 ovary. Kolreuter found that when 25 pollen-grains were placed on the 

 stigma of Hibiscus Trionum, 10-16 ovules were developed ; with 50 or 

 60 grains, above 30 ovules ; and 1,2, or 3 at the most sufficed for the 

 single ovules of Mirabilis Jalapa and M. lonyiflora. 



The bundle of pollen-tubes proceeding from the style is distributed in 

 fractions, or partial bundles, to the placentas, when several of these exist. 

 The pollen-tubes make their way to the points of the ovules (figs. 611, 

 p t, & 609, d, p ), and one or two enter the micropyle of each. Gene- 

 rally speaking, the tube ceases to elongate when it reaches the outer sur- 

 face of the apex of the embryo-sac. Sometimes it runs onwards a little 

 way (fig. 609, #), often depressing the membrane of the embryo-sac a 

 little. According to Hofmeister, it actually breaks through into the 

 embryo-sac in Canna. In all cases it contracts a firm adherence, and 

 possibly a kind of conjugation takes place (fig. 612, B). The end of the 



