39 



FOURTH GROUP. SEED-PLANTS. 



Fertilisation^. The pollen-grains which germinate on the stigma send their 

 tubes through the canal of the style, when there is one, or more commonly through 

 the spongy conducting tissue in the interior of the solid style, clown into the cavity of 

 the ovary 2 ; the micropyle often lies so close to the bottom of the style, both in erect 

 basal ovules (Fig. 308) and in pendulous anatropous ovules, that the descending 

 pollen-tube can enter it at once ; but in the majority of cases the pollen-tubes must 

 continue to grow on after their entrance into the cavity of the ovary in search of the 

 orifices of the ovules, and they are led in the right way by various contrivances ; such 

 are the papillae frequently found on the placentas or on other parts of the wall of the 

 ovary, along which the tubes advance ; in our European Euphorbias a tuft of hairs 

 conducts them from the base of the style to the neighbouring micropyle ; in the 

 Plumbagineae the tissue of the style forms a descending conical projection which 

 serves as conductor of the pollen-tube to the micropyle ; and other similar aids may 

 be observed. 



As each ovule requires a pollen-tube for its fertilisation, the number of tubes 

 entering the ovary is proportionate on the whole to the number of ovules which it 

 contains ; still the number of the tubes which enter the ovary is generally larger than 

 that of the ovules ; where these are very numerous, the number of the tubes is also 

 large, as in the Orchideae for instance, where they may be seen with the naked eye 

 in the ovary as a bundle of white silky threads. 



The time which elapses between pollination and the entrance of the pollen-tubes 

 into the micropyle depends not only on the length of the route which is often 

 considerable, as in Zea and Crocus, but also on specific characters in the plants ; thus 

 according to Hofmeister the pollen-tubes of Crocus vernus require only from twenty- 

 four to seventy-two hours to pass through the style which is six to ten centimetres in 

 length, while those of Arum maculahtm, which have a distance of scarcely two or 

 three millimetres to traverse, take at least five days, those of the Orchideae ten days 

 or even weeks and months, during which time the ovules are being developed in the 

 ovary or in many cases only begin to be formed. 



The pollen-tube is usually very narrow and thin-walled while it is rapidly 

 lengthening; when it has made its way into the micropyle its wall soon becomes 

 very considerably thickened, chiefly, it would appear, through swelling, so that the 

 lumen is only a narrow canal, as is the case in the Liliaceae, Cactaceae and Malva ; 

 Hofmeister compares it in this state to the tube of a thermometer ; sometimes the 

 lumen also enlarges, as in (Enothcra and Cucurbitaceae. The pollen-tube contains 

 a granular protoplasm, usually with numerous grains of starch. The nuclei of the 

 pollen-grain are according to Strasburger's account dissolved in the tube, in Orchis 

 only after the apex of the tube has reached the embryo-sac 3 . 



1 Besides the works cited above, see Hofmeister's Historical account in Flora, 1875, p. 125, 

 which gives the literature of the subject, and Strasburger, Ueber Befrnchtung u. Zelltheilung, Jena, 

 1878, [and Neue Unters. ii. d. Befr. b. d. Phanerog. Also D'Arcy Thompson, Catalogue of Books 

 and Papers relating to the Fertilisation of Flowers, 1883]. 



2 Regarding the passage of the pollen-tube see references in note on page 306. 



3 The analogy of the Gymnosperms would lead us to suspect that the nucleus of the pollen- 

 tube is not dissolved in the Angiosperms. but takes part directly in the fertilisation. 



