THE ANGIOSPERMAE 1441 



Pollen tubes do not always, however, enter by the micropyle. This, 

 which is the normal mode of entry, is called porogamy. When the micro- 

 pyle is not used the method is called aporogamy. Under the latter heading 

 are three variations: basigamy (chalazogamy) when the pollen tube 

 enters through the funicle and the chalaza; acrogamy, when the pollen 

 tube grows to the micropylar end of the sac but does not enter through the 

 micropyle; and mesogamy when the pollen tube penetrates the ovule from 

 the side. 



Aporogamy may arise from other causes than the behaviour of the pollen 

 tube; for instance in plants like Torenia where the embryo sac grows out 

 of the ovule, or the opposite circumstance, where the micropyle is com- 

 pletely occluded {Houstonia, Gunner a, Ficus carica). These cases are acro- 

 gamous but not porogamous. Aporogamy can occur, however, even where 

 there is an open micropyle, through the completely endotropic growth of the 

 pollen tube, as in chalazogamy. 



Chalazogamy was discovered by Treub in 1891 in Casuarina. The pollen 

 tube enters the chalaza through the funicle, sends out in the chalaza many 

 irregular branches and then makes its way upwards among the many elon- 

 gated embryo sacs, the upper end of one of which it finally enters. 



Similar behaviour has been discovered in a number of other plants, 

 notably in the amentiferous genera Betnla, Corylus, Carpiniis , Juglans , Carya, 

 Pterocarya and Ostrya, all members of the Betulaceae or Juglandaceae. The 

 Fagaceae and Myricaceae are all porogamous. The limited distribution of 

 chalazogamy in families held by some to be primitive gave rise to the belief, 

 suggested by Navaschin, that it was the primitive type of fertilization. This 

 idea, never very acceptable, has been countered by the discovery of occa- 

 sional chalazogamy in a number of plants of other families not markedly 

 primitive, i.e., Anacardiaceae. Against Xavaschin's theory, Murbeck has 

 argued that it is not possible to draw a line betw^een " true " chalazogamy 

 and other cases, which are widespread, for example among Rosaceae, in 

 which the pollen tube enters by the funicle but penetrates to the embryo sac 

 by way of the integuments rather than through the apex of the nucellus. 

 According to his view, chalazogamy is only a special case of completely 

 endotropic growth of the pollen tube, which is intercellular throughout its 

 length. This is a physiological peculiarity, found in many families and of 

 no phylogenetic significance. 



Alesogamy, if it is possible to distinguish it as a separate process, is 

 exemplified by Populus. The pollen tube enters the ovule through the side 

 of the integument nearest the placenta, curves round the channel of the 

 micropyle and reaches the embryo sac through the nucellus. 



The course of endotropic pollen tubes, especially in the ovular tissues, is 

 very irregular, indeed erratic, and neighbouring tubes may sometimes be 

 seen growing in opposite directions, which does not suggest any chemotactic 

 control from the direction of the embryo sac. 



