GYMNOSPERMAE. CONIFERAE. 



335 



Fertilisation. The pollination of the macrosporangium takes place before the 

 archegonia begin to be formed in the prothallium. The pollen-grains (microspores) 

 which have reached the apex of the nucellus thrust their tubes at first only a little way 

 into the tissue of the nucellus and then remain inactive for a time; when the 

 archegonia are fully formed in the prothallium, the pollen-tubes begin to grow again 

 in order to reach them. In Gingko biloba fertilisation takes place in October in the 

 macrosporangia (seeds) which have 

 fallen ripe from the tree, and the em- 

 bryo forms in the seed during the 

 winter. In the rest of the Coniferae 

 in which the seeds ripen in the year 

 this interruption in the growth of the 

 pollen-tubes only lasts for a few weeks 

 or months ; where the seed takes two 

 years to mature, as mfuniperus sibi- 

 rica,J.commums t Pinus sylvestris, and 

 P. Strobus, it continues till June of the 

 ensuing year. In the Abietineae and 

 Taxineae each pollen-tube fertilises 

 only one archegonium, several pol- 

 len-tubes therefore enter at the same 

 time ; in the Cupressineae on the 

 other hand one tube is sufficient for 

 the whole group of archegonia at the 

 end of the broad funnel of the pro- 

 thallium ; the pollen - tube fills the 

 funnel completely and lays its broad 

 extremity on the necks of them all. 

 Short narrow protuberances on the 

 extremity of the tube now grow into 

 the separate archegonia, thrusting the 

 neck-cells apart and destroying them, 



and they finally reach the OOSphere. FIG. 261. TOXUS canadensis. A longitudinal section through the 



rr-n . M v A i_ upper extremity of the prothallium (endosperm) ee and the lower extremity 



1 he prOCeSS IS Similar in the Abie- of the pollen-tube/;ar the archegonia, d their neck-cells; the archego- 



, 

 and 



, , , nium to the left is fertilised (June sth). B portion of the endosperm 



, Where the tUDe with an archegonium, the elongating suspensor - cells of which v are 

 lready in an advanced state of development, with the embryo at the 



, . , , j , , , n avance sae o eveopmen, w e emryo a te 



WhlCh had beCOme broader grOWS extremity; /the pollen-tube (June ioth). C longitudinal section of a 



nucellus on June isth ; kk nucellus, e endosperm, / pollen-tube, w two 



, , , - eus, e enosperm, oen-ue, w wo 



again and enters the neCK OI suspensors with embryos at their extremities proceeding from two 



oospores. N 



i . i -I . . uospores. iNumerous vacuoies are visible in tne oospneres ot the 



One arChegOniUm Only, making itS archegonia; the nucleus of the oosphere is not given. After Hofmeister. 

 i , i , , , A A magn. 300, B 200, C 50 times. 



way ultimately to the oosphere. A 



thin spot, a pit, may be recognised at the apex of the protuberance on the thick- 

 walled pollen-tube, and evidently facilitates the transmission of the fertilising substance ; 

 probably the pressure of the superior tissue on the portion of the tube outside the 

 archegonium assists the operation. The processes in the pollen-tube are thus 

 described by Strasburger : The small vegetative cells, that is, the cells of the 

 prothallium, in the pollen-grain (microspore), one or more in number, take no part in 

 the formation of the pollen-tube, but the nucleus of the large cell moves to the apex 



