THE ENDOSPERM 259 



not in others. Indeed, these effects are now so clearly understood 

 that there is hardly any reason for continuing to refer to them by the 

 obscure and somewhat confusing term xenia. 



More difficult to interpret is the so-called metaxenia, i.e., the 

 effect of pollen on the maternal structures (seed coat or pericarp) 

 lying outside the embryo sac. The most important work in this 

 connection is that of Swingle (1928) who finds that in the date palm 

 (Phoenix dactylifera) the time of maturity of the fruits as well as 

 their size can be made to vary according to the type of pollen used 

 in fertilization. 14 Regarding the nature of the mechanism which 

 enables this to take place, he suggests that possibly the embryo or 

 endosperm or both secrete hormones, or substances analogous to 

 them, which diffuse out into the wall of the seed and fruit and exert 

 a specific influence on them, varying according to the particular 

 male parent used in the cross. 



Although it is not inconceivable that pollen may sometimes 

 exercise such an influence on the tissues of the ovary and conse- 

 quently on the shape, color, or flavor of the fruit, it seems that other 

 factors should also be taken into consideration before arriving at 

 such a conclusion. Differences in size and shape of fruits may also 

 be caused by the number, state of maturity, and genetic constitution 

 of the seeds which develop inside them. To mention a single ex- 

 ample, in certain apples self-pollination gives seedless or nearly 

 seedless fruits which are ribbed towards the apex and have a greater 

 height than breadth. On the other hand, the fruits formed after 

 cross-pollination have many seeds, their shape is much more sym- 

 metrical, the height and breadth are almost equal, and the ribbing 

 is scarcely noticeable (Crane and Lawrence, 1947). It must, there- 

 fore, be concluded that while Swingle's explanation may be correct, 

 we do not yet have a sufficiently extensive or critical set of observa- 

 tions to afford a clear insight into the matter. 



Mosaic Endosperm. A very interesting condition, occasionally 

 encountered in some plants, is the lack of uniformity in the tissues 

 of the endosperm. In Zea mays, patches of two different colors 

 have sometimes been observed, forming a sort of irregular mosaic 

 pattern, or part of the endosperm is starchy and part is sugary. 

 Webber (1900), who observed an intermingling of red and white 

 color in Zea mays, proposed an ingenious explanation to account for 



14 Similar reports have also been made for a few other plants (see Harrison, 1931; 

 Nebel, 1936; Schreiner and Duffield, 1942). 



