92 CARNEGIE INSTITUTION OF WASHINGTON. 



abnormal leaves and almost entirely sterile flowers. Dr. Belling reports the 

 pollen mother-cells of this plant apparently to be (2w— 1). 



Sectorial chimeras appear to be a not infrequent stage in the production 

 of tetraploid races. In 3 plants otherwise diploid, a branch was found to be 

 tetraploid (4n), as indicated by pollen-size. The growth of these 4n branches 

 was slow and buds were not secured for chromosome counts, but seeds from 

 the 4n branches produced 4n offspring, while seeds from the 2n branches 

 produced 2n seedUngs. Tetraploids frequently first appear in a peculiar 

 rough-leaved form in which patches are devoid of palisade parenchyme, and 

 it is possible that, in addition to the sharply defined sectorial chimeras, 

 periclinal chimeras and chimeras with a less orderly mixture of chromo- 

 somally distinct tissue may occur in Datura. 



Rate of Pollen-Tube Growth in Datura Mutants. 



Irregularities in some proportions obtained in certain crosses between the 

 varieties of Datura have led to a study, by Professor John T. Buchholz, of 

 the relative rate of pollen-tube growth of different varieties, following along the 

 lines of an hypothesis suggested by Correns. This study, made during two 

 summers, has yielded important results described by Dr. Buchholz as follows : 



"A study of the germination and growth of normal pollen on the stigmas of 

 a dozen or more simple trisomic mutants of Datura on cut flowers under com- 

 parable conditions (about 18° to 19° C.) has shown that there is no great 

 difference in the depth of penetration of the pollen-tubes in the different cases, 

 though it is somewhat slower as a rule for most of the mutants. Stigmas of 

 normal plants were used under the same conditions for the germination of the 

 pollen of 18 of these (2w + l) chromosomal mutants, with the result that the 

 pollen differed widely in the percentage of its germination, as well as in the 

 depth of penetration of the pollen-tubes. The pollen-tubes having n chromo- 

 somes from each mutant penetrate to about the same distance as normal 

 pollen-tubes, while there seems to be a distinct group of slower pollen-tubes, 

 presumably those having the (n+1) chromosomes, which either fail to ger- 

 minate or lag behind. In a few of the mutants, such as Sugarloaf, for 

 example, there seems to be a distribution of these pollen-tubes into three 

 modes, suggesting the possibihty that there may be more than two classes of 

 pollen produced, while pollen-tubes from normal plants are distributed essen- 

 tially in a unimodal curve. 



"Considerable attention was given to a preliminary study of the physiology 

 of pollen-tube growth in normal plants. Pollinations with second batches of 

 pollen applied to used pistils after intervals of 1, 2, and 3 days show that the 

 second pollen- tubes are retarded to about two-thirds to one-half of the growth- 

 rate of the same pollen on the pistils of fresh normal flowers. A similar appli- 

 cation of normal pollen to the pistils of unpoflinated flowers collected at the 

 same time and stored beside them under the same conditions shows that this 

 aging through storage also slows down the depth of pollen-tube penetration to 

 some extent, but the effect is much less than that produced by the previous 

 growth of other poflen-tubes. These experiments suggest that pollen-tubes 

 deplete the style of nutrient substance or render it slightly toxic, possibly both. 



"From a series of experiments performed last year, it was definitely proved 

 that the fertihzation of ovules proceeds downward from the upper part of the 

 ovary, that most of the seeds in the upper half of the seed capsule were fer- 

 tilized by the first pollen-tubes arriving in the ovary, and most of the seeds in 

 the lower portion of a capsule were the result of fertihzation by the slower or 

 later-arriving pollen-tubes." 



