1436 A TEXTBOOK OF THEORETICAL BOTANY 



intensity, from the death of pollen or stigma or both, to a mere slowing down 

 of the rate of growth of the pollen tubes. These incompatibility reactions 

 have great genetic importance, which we shall speak of in Volume III, but 

 cannot go into further here. 



An interesting example of such a reaction is seen in the following experi- 

 ments. Pollen of the Peach variety " Elberta ", when germinated on artifi- 

 cial media, gives pollen tubes of two lengths. In 50 per cent, the mean length 

 was 141 •751J. and in 50 per cent, it was 2345 ^Sa. The difference is factorial and 

 is due to the presence of an inhibitor in the short-tube grains. The same 

 thing is found in the Apple " Landsberger Renette ", but not in all trees. 

 Crossings of this Apple were made with " Beauty of Boskoop " from trees 

 with uniform pollen and from trees with heterogeneous pollen. The former 

 gave successful fertilization, the latter were wholly sterile. This means that 

 the 50 per cent, of grains giving long tubes were also inhibited on the stigma. 

 They must also have contained an inhibitor, activated by interaction with 

 the stigma, but inactive in culture. 



In incompatible crosses the pollen tube growth is generally much slower 

 than in fully compatible pollinations. Many tubes do not reach the ovary. 

 The pollen may germinate normally and the effect becomes progressively 

 evident thereafter. Pollen tubes become coiled and distorted. The inhibitor 

 is extractable from the stigma and an extract inhibits pollen tube growth in 

 cultures. There is a marked resemblance to an antigen-antibody reaction. 



The Pollen Tube 



The discovery of pollen tubes, or at least the recognition of what they 

 are, is credited to Amici in 1823, ^^ pollen of Portulaca. Certainly he was 

 the first, in 1830, to trace the pollen tube of Yucca gloriosa from the stigma 

 to the micropyle. The tube begins as a passive extension of the intine of 

 the grain but it quickly acquires a terminal growth of its own. The tube is 

 smooth-walled and cylindrical and the whole content of the pollen grain 

 passes into it, including the nuclei, the vegetative nucleus, henceforth called 

 the tube-nucleus, going first. The wall contains cellulose but the apex also 

 contains pectins and seems to be differently constituted from the rest, 

 judging by the ease with which it can be made to disgorge its contents in 

 artificial culture and, of course, in the embryo sac. The final volume of a 

 tube, in plants with long styles, may be many hundreds of times the 

 volume of the original grain, but only a small portion of this is occupied 

 by living matter. The protoplasm of the grain does not seem to increase 

 noticeably in the tube and is all concentrated in the forward end. Behind it, 

 at intervals, are formed callose plugs which cut off the older, empty part of 

 the tube. When the ovule is reached there is, therefore, no longer any living 

 connection with the pollen grain. The empty membrane does not last long 

 and the older parts may have disappeared even before the end reaches the 

 ovule. Such immense growth naturally requires nutrition. The tube 

 generally contains starch grains, received from the grain, as well as plastid 



