128 



PLANT STUDIES 



stigma of the same flower are not mature at the same time. 

 It is evident tliat this is a very effective method of prevent- 

 ing self-pollination. When the pollen is being shed the 

 stigma is not ready to receive, or when the stignni is ready 

 to receive the jiollen is not ready t(.) he shed. In some 

 cases tlu', pollen is ready first, in other cases the stigma, 

 the former condition being called proltDidriJ, the latter 

 j)r(iii)fjiiny. Tills is a very common method of preventing 



self-i3ollination, and is com- 

 moidy not associated with 

 irregularity. 



The ordinary ligwort nniy 

 l)e taken as an example of 

 protogynv. When theilowers 

 first open, the style, bearing 

 the stigma a.t its tiii. is found 

 Fio. 134. Flowers of ftre«c„i iE„i. pvotrudiug from tlie um-like 



to/«'««i), showingprotandry. In 1 the flower, wllilc tlie four 

 st.'uiieiit, are tlu'tist forward, and the , -i i 



, , . , , , , , , , stamens ai'c (Uii'vimI down 



style IS Kliarply InriU'd downward and 



ijaekwani. In 2 tlie style is thrust into the tiilic, aiul not ready 



forward, with ils slitrmatie branches i^ 1, ] tl ^;„ ,11 \ i- 



, ' . .■ . , , to shed llieii iiollcn. At 



spread. An insect in ]iassing Irotii 1 ^ 



to ■,' will almost eertainly transfer iHil- SOUK' later tiuiC tile Stvlc 



len frointheslanlensof llothestig- l,(,.^,.„i^, tllC stlo'lna Wilts, 



mas of ;,'.— Afler fillAV. ' '^ 



a.iid tlic stamens straighten 

 up and protrude from the tulie. In tliis wav, first tlie 

 recei)tive stignui, and afterwa.rds the shedding pollen-sacs, 

 (x.'iaipy t,he sa.me p<isiti(ni. 



I'l'otandry is even more conniion, and niaiiv ilhistrations 

 can lie ol)talne(l. h'or examjile, the showv (lowers of the 

 common fii'cweeil, or great willow lierb, Mdieii first opfjietl 

 ilisphiy I heir eight sheildiiig slaiiieiis |ironiineiitlv, the style 

 being sharply curved clowiiward a.ii(l back^^■a.^d, cari'yiug 

 the four stigma, lol)es well out. of the wav. Later, the 

 stamens l)end awav, and tlie style straightens up and ex- 

 ]ioses its stigma lobes, now receptive (see Fig. i;!l). 



(3) Difference in pollen. — In these cases tliere tiro at 



