412 



FERTILIZATION AND FORMATION OF FRUIT IN PHANEROGAMS. 



to the micropyles. In point of fact they follow a more roundabout course, creeping 

 along the ovary wall, and then up the individual ovules to the micropyle. 



It has been stated above that the pollen-tube enters the ovule at the T^xicropyle 

 in the vast majority of flowering plants. But this is not universally the case, as 

 recent investigations have proved. In several of the trees belonging to the group 

 of the Amentacese the pollen-tube follows an altogether different course. This 



was first established for the case of 

 Casuarina, a curious switch -plant 

 indigenous to Australia and certain 

 parts of Eastern Asia. A small por- 

 tion of a branch of this tree is shown 

 in fig. 69 ^ (vol. i. p. 299). Casuarina 

 is wind-pollinated, the flowers being 

 unisexual, though both male and 

 female flowers are borne on the same 

 individual. The pollen -grains ger- 

 minate in due course on the stigma, 

 and their tubes traverse the tissues of 

 the style. The tubes do not, however, 

 enter the cavity of the ovary, but con- 

 tinue their growth immersed in the 

 tissues of the ovary. They thus make 

 their way to the points of insertion 

 of the ovules, where they enter them, 

 and at once travel to the base or 

 chalaza. From the base of the ovule 

 the pollen-tube penetrates towards 

 the apex of the ovule, reaching the 

 egg-cell from below, not from above 

 as in cases in which the pollen-tube 

 entered the ovule by the micropyle. 

 This type of fertilization has been 

 termed chalazogamic in contradistinc- 

 tion to the more usual micropylar or 

 porogamic method. Quite lately a 

 renewed examination of many common 

 trees belonging to the Amentaceous 

 group has shown that in several of them also fertilization is chalazogamic. This is 

 the case in the Hazel {Corylus) and Hornbeam {Carinnus, fig. 314 a), belonging to 

 the Corylaceae, as also in the Birch (Betula) and Alder (Alnus, fig. 314 b), belonging 

 to the Betulacese. In the Hazel and Hornbeam the pollen-tube, after reaching the 

 base of the ovule, passes straight up to the egg-cell in the embryo-sac (cf. fig. 314 a, 

 p.t.) in a manner similar to Casuarina, but in the Birch and Alder its course is 



314A.— Chalazogamic fertilization in the Hornbeam 

 (Carpiniis Betulun). 



The drawing shows a longitudinal section of an ovule almost fill- 

 ing the cavity of the ovary. The micropyle (?n) and the two 

 integuments are shown above the apex of the nucellus (in 

 whose cells the nuclei are indicated). Within the nucellus 

 three embryo-sacs are represented; a pollen-tube (p.t.), pass- 

 ing down into the substance of the ovule from the placenta, 

 follows the course of the raphe and at the base of the nucel- 

 lus (chalaza) bends sharply round and enters one of the 

 embryo-sacs; its tip penetrates to the egg-cell at the apex 

 of the embryo-sac. Much enlarged (from a drawing by 

 M. F. Ewart). 



