REPRODUCTION. 647 



complete prothallium developed in the embryo-sac, on the 

 other. Hence, even in a cleistogamous flower, one, namely, 

 which does not open, so that the only pollen-grains which 

 can reach its stigma are those developed in its own anthers, 

 self-fertilisation, morphologically speaking, does not take 

 place, though, physiologically speaking, it does, as we shall 

 subsequently see. 



Even in monoecious Phanerogams dicecism is practically 

 attained in various ways. In some cases the structure of the 

 flower is such that, in view of the visits of insects which are 

 attracted by the colour of the perianth-leaves, or by the scent, 

 or by secreted nectar, the probability that foreign pollen, 

 derived at least from a different flower of the same plant, 

 will reach the stigma is very much greater than that its own 

 pollen will do so. A striking case of this is afforded by 

 heterostyled flowers, such as those of various species of Primula 

 and Oxalis, Lythmm Salicaria, etc. In other cases the 

 same end is attained by Dichogamy, that is, that the two 

 kinds of spores come to maturity at different times : in some 

 plants, termed proterandrous, the pollen-grains mature first, 

 in others {proterogynotis) the embryo-sacs. In either case it is 

 impossible that the pollen of any one flower should fertilise 

 the oospheres of the ovules of that flower. 



Proterandry is very common ; in fact, as Sir John Lubbock observes, 

 the greater number of flowers which contain both stamens and pistil, are 

 more or less proterandrous. The following are proterogynous ; Scro- 

 phularia nodosa, species of Plantago, Aristolochia, Arum, Euonymus, 

 many Rosaceae. 



Again, apart from any structural arrangements for ensuring 

 cross-fertilisation, there are in some cases imperceptible 

 physiological conditions which lead to the same result. It 

 is in some cases impossible for sexual reproductive cells of 

 nearly allied origin to fuse together. An indication of this 

 is afforded in Acetabularia and Ectocarpus by the fact that 

 conjugation can only take place between planogametes 

 derived from distinct gametangia, and it attains complete 

 expression in Dasycladus, among the lower plants, in which 

 conjugation only takes place between planogametes derived 



