708 Proceedings of the Royal Irish Academy. 



In a series of sections cut to 10 /a a single dividing nucleus of a vege- 

 tative cell may appear on two or even tlu'ee successive sections, and 

 when the poles of such a nucleus lie in a plane parallel to the surfaces 

 of the sections, the disposition of the chromosomes is very readily 

 followed. This occurrence, -which is by no means exceptional, renders 

 the counting of the chi'omosomes, which is, ordinarily, in free hand 

 preparations a difficult and tedious task, perfectly simple. Thus, fig. 

 1, PL XXIII., represents the portions of dividing nuclei found on 

 successive sections. In fig. 1 eight chromosomes are to be seen in 

 the upper aster, and seven in the lower, in the first section, and in 

 the next eight above and nine below. 



In these examples the number of chromosomes in the nucleus is 

 sixteen (or thii'ty-two after fission), and I found this number to occur 

 in the majority of the nuclei of the cells of the gi'owing region, but 

 there were also other nuclei in which the number was larger ; in many 

 instances twenty-four. In some cases only twenty or twenty -two 

 could be seen. The smaller numbers may be due to the fact that a 

 few chromosomes were in these cases extravasated by the action of the 

 knife, although I think it probable that some of the nuclei may possess 

 these numbers of chromosomes which are intermediate between the 

 extremes sixteen and twenty-four. Nurubers greater or less than these 

 were not found. The cells containing the twenty-four- or sixteen- 

 valent nuclei did not appea-r to occupy any definite position in the 

 plant, nor is the one or the other kind of nucleus characteristic of the 

 cells belonging to any definite tissue-system, e.g., nuclei with twenty- 

 four chromosomes were found in the dermatogen of the stem and young 

 leaves, in the cortex, in vascular strands of the leaf, in the central 

 medullaiy tissue of the stem, in the mesophyll, in the subepidermal 

 layer of the young anther. The nuclei with sixteen chromosomes are 

 about twice as niunerous as those with twenty to twenty-four, and 

 they are found in the same organs and systems of tissue as the latter. 



The possibility that the number of chromosomes may vary in the 

 asexual cells of a plant has already been expressed by Strasburger 

 (Ann. of Botany, vol. viii. Ifo. xxxi, 1894), and Guignard (" jSTouv. 

 Etud. s. 1. Fecondation," p. 246), and from my observations on 

 L. longiflorum it appears not to be an accidental occurrence but rather 

 the rule in this plant. Sti'asburger, however {Joe. cit.), is inclined to 

 believe that such variations do not occur in the embryonic tissues or in 

 the growing region, but it was in these latter positions in which most 

 of my countings were made, and from my preparations I can hardly 

 doubt but that the number is in this case, at least, variable. 



