312 



GYMNOSPERMS 



The microspore in conifers, as in cycads, has a very definite or- 

 ganization. The difTerentiation into apex and base is just as definite 

 as the sporophyte differentiation into root and shoot. The be- 

 havior, as far as apex and base are concerned, is exactly the opposite 

 of that in cycads and Ginkgo. In the cycads and Ginkgo the pollen 

 tube functions principally as a haustorium — ^doubtless, the original 

 function of a pollen tube — ^while the prothallial end of the tube, with 

 the whole pollen grain, grows down into the nucellus, and this pro- 

 thallial cell end of the tube is the one which ruptures and sheds the 



Fig. 314. — Abies balsamea: pollen grain at time of shedding; b, body cell; s, stalk 

 cell; p, prothallial cell; pi, starch; /, tube nucleus; X535. 



gametes. In conifers, the prothaUial cell end of the tube, with the 

 entire pollen grain, remains where it alights upon the nucellus, and 

 the tube grows down into the nucellus, serving as a haustorium, but 

 also as a carrier of the gametes. 



In the Araucariaceae, as shown in figs. 312 and 313, the generative 

 cell divides and forms the stalk and body cell before the pollen is 

 shed. This is also true of some others with extensive prothallial 

 tissue, as in Podocarpus, Dacrydium, and Phyllocladus. This division 

 occurs regularly in Ahies and Tsuga] but here the stalk cell has a 

 typical stalk position (fig. 314). In these two cases, one or both of 

 the two prothallial cells may divide, so that there may be three or 

 four prothallial cells. In the Abietaceae there is usually a consider- 

 able interval between the formation of the first and second pro- 

 thallial cells, so that the rapidly thickening intine overgrows the 



