iS8 LIFE : orTLlNFS OF GFNERAL BIOLOGY 



Kock Rose, ami Balsam. In a f«'\v cases, like the Bee Orchis {Ophrys 

 apif^ra). all the llowers are cleistofjamous. The intimacy of the 

 self-fertilisation may \yc inferred from the fact that in some cases, 

 like sorrel and balsam, the anthers of the stamens do not split, and 

 the |)ollen-tulH^ has tu grow out of the anther into the pistil. In many 

 cases there is a great reduction in the carix'ls, e.g. the absence of a 

 stigma, and a great reduction in the number of the stamens. This 

 peculiar occurrence of closed self-pollinating flowers may be regarded 

 as an arrest of development a time-variation— \n conditions of 

 unusual difticulty. 



It should Ik- noted that autogamy does not necessarily mean that 

 the flower itself effects the deix)sition of its own pollen on its own 

 stigma, for that may Ix^ effected by insects within the flower. The 

 well-known Yucca-moth. Bronuba. seems often to pollinate a stigma 

 of the Yucca flower with a ball of pollen collected from the satm 

 bloom; but sometimes the ]K)llen is carried from another Yucca plant, 

 and that is what is meant by cross-}X)llination. Intermediate between 

 the two, but nearer autogamy, are cas^\s where the jx)Ilen is brought 

 from another flower of the same plant. The Yucca-moth may be an 

 agent in all the three modes of pollination— within the same flower, 

 within the same inflorescence, or from one Yucca plant to another! 



WIND POLLINATION. -In many conifers, like the pines, and 

 in catkin-lxaring trees, such as birches and poplars, and also in 

 many grasses, the jx)llen is carried from plant to plant by the wind. 

 In such cases it is common to find one or more of the following 

 adaptations, mobile inflorescences easily shaken by the wind; 

 very open and simple flowers which are often ripe before there is 

 much foliage; delicate filaments to the stamens so that the anthers 

 (occasionally e\])I()sive. as in nettles) are readily jostled in the breeze ; 

 abundant ix)llen. exce]>t in some of the grasses and sedges; dry 

 pf)llen grains (.sf)metimes with wings, as in pines); and plumose 

 stigmas which e.xpose a large surface for jxillen capture. Many wind- 

 IK)llinated plants, such as oak-trees and grasses, are con.spicuously 

 successful, but ixrha]>s there is no correlation between the two facts. 

 rher<' seems not a little wastefulness and fortuity in wind-pollina- 

 tion; yvi it is well suited for plants with primitive or slightly 

 develojvd |Ktals and s^jials. and for those who.se flowers appear 

 early in the year. Ixfore there are many insects about. Self-pollina- 

 tion is often avoided in wind-jx)llinated and in other flowers by the 

 fact (dichogamy) that the stamens and pistil are not ripe in the same 

 flower at the same time. The ]>rotandrous condition, when the 

 stamens are rijx' first, is illustrated liy ("ampanulas. Composites, 

 Labiates. I'mlx Ilifers. etc.; the ]>rolo;^'ynous condition, when the 

 pistil is rijx- first, is illustrated by l-oxtail Grass, Sedges, Figs, 

 Hellelx>re, Hops. etc. 



