THE ANGIOSPERMAE 1437 



rudiments and other enclosures, but there seems Httle doubt that the 

 tube takes up nutriment from the cells it contacts during growth in the 

 style. 



The rate of growth in pollen tubes is very variable. Rates varying from 

 less than i mm. to 5 or 6 mm. per hour are recorded but these measurements 

 were often in cultures and made without regard to temperature, which has 

 a most marked effect. Thus in Datura stramonium at iii C. the average 

 rate of growth was i 28 mm. per hour, while at the temperature optimum 

 of 33"3°C., the rate was 5-86 mm. per hour, or four and a half times as 

 great. In barley at 30-35^0., fertilization was accomplished in 20 mins. 

 from the time of pollination, but at 5'C., 140 mins. was required. Grass 

 pollen which has generally the shortest life also grows the fastest. Important 

 and enormous variation also exists in the time elapsing between pollination 

 and fertilization under natural conditions. Times of a few minutes are 

 recorded for some Gramineae, while in Amentiferae, as in Coniferae, the time 

 extends to months (Betula, one month; Corylus, four months; Oiiercus 

 veJutina, more than one year). The same is true of many Orchids, although 

 in Orchis it is less (8-20 days). Periods between one and five days are fairly 

 common. The time is not related to the length of the style; compare the 

 times in Amentiferae, given above, with Crocus, 24 hours; his, 79 hours; 

 and Zea, 25 hours, all plants with exceptionally long styles or stigmas. 



The long delay between pollination and fertilization which is found in 

 the most various families is a matter of some biological interest. In many 

 cases it is due to the imperfect development of the ovule at the time of polli- 

 nation. 7'he pollen tube grows as far as the nucellus and there awaits the 

 ripening of the ovule. In Hamamelis it gets only as far as the funicle and 

 there it passes the winter, completing its work the following spring. 



In the Orchidaceae not even the placentae are formed at the time of 

 pollination, which apparently is a necessary stimulus for the completion of 

 carpels and ovules. 



Normally each pollen grain emits one pollen tube only, but multiple 

 tubes arise in some pollen which is provided with numerous pori, especially 

 in Malvaceae, Cucurbitaceae and Campanulaceae. The pollen nuclei enter 

 one of the tubes formed and the others soon stop developing, but they may 

 perhaps function temporarily as haustoria. This idea is supported by the not 

 infrequent branching of the ends of pollen tubes to form haustoria in the 

 ovule. Pollen tube haustoria are conspicuously shown in Cucurbita pepo. The 

 nucellus in this plant has a long beak which fills the endostome and later 

 becomes suberized. When the pollen tube meets this beak it enlarges into a 

 broad vesicle from which spring a number of branches (Fig. 1320). One 

 of these penetrates and partially destroys the nucellus and performs fertiliza- 

 tion; the others then spread laterally into the integuments. The inner layer 

 of the outer integument is made up of cells rich in starch and protoplasm 

 and in this the pollen tubes ramify. Nourishment for the growing embryo and 

 endosperm is thus conveyed through the vesicle and this continues until the 

 seed is ripe. That there is a true haustorial function here rests on deduction 



