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EMBRYO, SEED AND CARPEL IN THE DATE. 129 





The facts observable with the microscope seem to force the 

 conclusion that the mechanical conditions determine the be- 

 havior. Thus, in certain Rubiaceae, the embryo-sac extends 

 forward along the micropyle, during the development of the 

 single thick integument. If, as may happen, instead of 

 remaining within normal bounds, it continues to grow until 

 it extends beyond the exostome, it then pushes its way in 

 any direction — the easiest mechanically. But if, instead of 

 moving forwards, it develops backwards into the chalazal 

 tissue, its path is indirect, and appears to be wholly indeter- 

 minate. In the date, however, the backward growth of the 

 endosperm is direct and determinate. To be sure, it is 

 pointed in the right direction to begin with. Though it 

 digests the nucellus — a very thin layer — and to this extent 

 moves forward, it is resisted by the integumental tapetum. 

 So far the same phenomenon is seen in other plants 

 (Elaeagnus, Servettaz, I. c. Cucurbitaceae, Kirkwood, 1904) 

 but to a more marked degree. This resistance may, 

 probably, be explained by a chemical response, or by 

 a resistant layer, say a cuticle, as observed in Tricyrtis by 

 Ikeda (I. c). But how shall we explain the direction of 

 the antipodal haustorium, unless we assume, as a working 

 hypothesis, that the response is chemotactic and that, there- 

 fore, many facts about the embryo-sac are to be explained 

 in the same way that we explain the behavior of the pollen 

 tube! To assume a local enzymatic activity would be, I 

 think, to assume too much, though this also is not impossible. 

 Later stages (the young seed). During the whole period 

 of development until maturity the raphe and integuments 

 contain iron-blue tannin. For some time — no exact period 

 can be determined — the reaction in the integuments is 

 diffuse, as it is in the bulk of the raphe, in which, however, 

 are groups of cells showing a superior content. The appear- 

 ance of these cells suggests that the tannin in them is aplastic, 

 while their position, which is removed some distance from 

 the endosperm, also indicates this to be the case. Similarly, 

 as stage III is approached, there is evidence below the level 

 of the embryo, that the tannin is accumulating in permanent 





