THE ANGIOSPERMAE 1273 



may occur in the same flower. Autogamy pure and simple is comparatively 

 rare except in cleistogamous flowers, but it often occurs secondarily as a 

 concomitant of allogamy, being, as it were, a safety mechanism which 

 ensures that self-pollination shall occur if the biologically preferable cross- 

 pollination should tail. 



Some chasmogamous flowers with a structure from which one would 

 infer allogamy have nevertheless adopted the habit of self-pollination, as if 

 by retrogression they had abandoned allogamy and adopted autogamy as a 

 short cut. This is the case in the Sweet Pea, whose stigmas are already 

 receptive, and pollinated, before the flower opens. Some of the cereal 

 Grasses also, although members of a family which habitually uses wind- 

 polHnation, are self-pollinated in the bud state. Even if cross-pollination 

 occasionally occurs at a later stage in such flowers, the start obtained by the 

 pollen-tubes of the flower itself would practically ensure that self-fertiliza- 

 tion resulted. 



In cold northerly climates, where insects are very scarce, autogamy is 

 prevalent among many plants which are allogamous in more favourable 

 regions. In such climates Diptera may be locally plentiful near human 

 settlements and they may serve as pollinators for a number of plants usually 

 dependent on bees, e.g., Thymus, Geranium, Melandrium and Iris. This 

 pollination by flies may be allogamous but is more generally autogamous. 

 Gunthart observed that in Cruciferae and Crassulaceae and in the genus 

 Saxifraga, all entomophilous species were also capable of autogamy and 

 that the nature of the pollination might vary with the locality, protogyny 

 changing into protandry and vice versa. 



Obviously, without self-fertility autogamy is useless, and it may fre- 

 quently occur without result. Self-fertility does not seem to cause any 

 debilitation of the plant, but it must very seriously limit the gene-content 

 and impoverish the plant from the point of view of variation, except for the 

 comparatively rare occurrence of gene mutations. 



Autogamy may be brought about in a great many ways. Indeed there 

 are almost as many devices working for autogamy as for allogamy. One of 

 the simplest principles is that of an overlap in time between the ripening 

 of stigmas and anthers in dichogamous flowers, without any other arrange- 

 ment. There is thus provided a period during which self-pollination is 

 possible and usually occurs, if the anthers are close to the stigma. This is to 

 be seen in many Liliaceae, Amaryllidaceae and Iridaceae and in some 

 Dicotyledons such as Geranium and Lithospermum. The principle of an 

 overlap in time is indeed inherent in the following more complex arrange- 

 ments, since without it autogamy would be difficult and rare. 



Movements of the floral parts often assist autogamy and these may be 

 divided into movements of growth and movements due to bending or folding 

 of organs. In the first category we have those upright flowers where the 

 stigmas are at first above the anthers and so out of the way of their pollen. 

 Subsequently, however, the stamen filaments may elongate and place the 

 anthers either beside or above the stigmas, which are still receptive. This 



