392 



FOURTH GROUP. SEED-PLANTS. 



the transference is accomplished according to Strasburger in the following manner. 

 In a suitable subject, as in Torenia asiatica, the pollen-tube may be seen to attach 

 itself firmly to the synergidae as soon as it has reached them, and breaks before it 

 will allow itself to be separated from them. Then the contents of one of the syner- 

 gidae become turbid, its nucleus (?) and its vacuole disappear, its protoplasm becomes 

 finely granular and afterwards very strongly refractive, and now agrees entirely with 

 the contents of the pollen-tube in density, granular character and colour. The 

 companion cell either goes through the same changes or takes no part in the process 

 of fertilisation. The synergidae next lose their form, and pieces of them break away 

 and attach themselves to different parts of the oosphere, which must take into itself 

 shapeless portions of one or both synergidae, for its contents become richer in 

 granular substances, and it appears to be invested with a cell-wall as a result of 

 impregnation. In cases favourable for observation, as in the Orchideae and Monotropa, 

 two nuclei can be perceived in the oosphere after impregnation, one of which is 

 certainly formed of matter from the pollen-tube (the sperm-nucleus), while the 

 other is the nucleus of the oosphere. The two nuclei coalesce, and then the oospore 

 which is already invested with a cell-wall begins to develope into the embryo. The 

 pollen-tube remains closed during the process of impregnation; it must therefore 

 remain uncertain in what form the fertilising substance passes. The one of the 

 synergidae which is not employed in the impregnating process persists for a time and 

 ultimately disappears like its companion, and the pollen-tube also after a time can no 

 longer be recognised. 



Fertilisation is usually accom- 

 plished in a very short time after the 

 pollen-tube reaches the apex of the 

 embryo-sac ; yet the cases are not few 

 in which a long time elapses between 

 the arrival of the pollen-tube and the 

 commencement of the development 

 which it excites ; several days or weeks 

 in woody plants as Ulmus, Quercus, 

 Fagus, Juglans, Citrus, jEsculus, A er, 

 Cornus, Robinia, almost a year in the 

 American oaks which take two years 

 to ripen their seed ; in Colchicum 

 autumnale the pollen-tube reaches the 

 embryo-sac at the latest by the beginning 

 of November, and it is not till the May 

 of the next year that the embryo begins 

 to form (Hofmeister) *. 

 Even the penetration of the pollen-tube into the conducting tissue of the style and 

 to the cavity of the ovary often causes extensive changes in the flower ; if the perianth 

 is delicate it usually loses its turgidity at this time, withers, and soon falls to the ground ; 

 it is common in the Liliaceae for the ovary to begin to increase rapidly in size before 



FIG. 322. Funkia cordata. A apex of the embryo-sac e covered 

 by a layer of cells of thenucellus KK\ xone of the synergidae, beside 

 it the peculiarly shaped oospore with its onucleus. B, C ospores 

 before division E after division. F the pro-embryo differentiating 

 into the spherical suspensor-cell and the two-celled rudiment of the 

 embryo. Magn. 550 times. 



Hofmeister, Neue Beitr. (Abh. d. K. saehs. Ges. d. Wiss, Bd. VI. and VII). 



