ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 561 



the thallus is differentiated into very primitive water-conducting 

 strands. 



In the rhizome of the Polytrichacea3 there is a much stronger 

 differentiation. The authors propose, for the water-conducting tissue as 

 a whole, the term hydrom, the water-conducting cells themselves being 

 hydroids. The group of sieve-tube-like cells are termed leptoids, these 

 being surrounded by a starchy parenchyme or amylom. The tissue- 

 system of the rhizome of the Polytricbacese is practically identical with 

 that of the root of a vascular plant ; but the pericycle and endoderm 

 are less complete. The tissues of the central cylinder may be classified 

 into hydrom, leptom, and conjunctive tissue (stereom and unlignified 

 cells with or without starch). They are marked by an entire absence 

 of pits. In the aerial stem the highly developed stele is essentially 

 double in nature, consisting of a central primitive hydrom-cylinder, 

 originally developed, and still serving to supply the apical bud, sexual 

 organs, and sporogone with water ; and of a double peripheral mantle of 

 hydrom and leptom separated by a starchy hydrom-sheath (amylom) ; 

 all three layers being composed of the joined traces of leaf-bases. The 

 stem-stele and leaf-bundles, while originating quite independently, 

 form together a complicated conducting system for carrying water to 

 the leaves and carrying away from them the formed food-material. 



Nuclear Division in Pellia.* — Prof. B. M. Davis has carefully 

 followed out the processes connected with the division of the nucleus in 

 the formation of the spores in this genus of Hepaticae. The following 

 are among the more interesting results. 



At the time of sporogenesis the nucleus comes to occupy the geo- 

 metrical centre of the 4-lobed spore mother-cell, and there passes 

 through synapsis previous to the first mitosis. Emerging from synapsis, 

 the nucleus presents a delicate spirem-thread, which soon segments into 

 the chromosomes. The first indication of spindle development appears 

 in a zone of granular kinoplasm closely investing the nucleus. There 

 is never, as has been stated, a true quadripolar spindle. There are two 

 successive mitoses in the spore mother-cell, with a short period of rest 

 between them. The poles of the spindles are never occupied by bodies 

 comparable to centrospheres or centrosomes. The rod-shaped chromo- 

 somes split longitudinally in each mitosis. Following the mitoses in 

 the spore mother-cell, each nucleus takes up a position in the centre of 

 the spore, assuming a resting condition. They increase in size, and 

 soon prepare for division, passing the synapsis. Two centrospheres 

 with radiations resembling asters are organised during the prophase, 

 and these develop the poles of the spindle ; but the author has not 

 been able to satisfy himself that the centrospheres were permanent 

 structures, or that they ever divided. There is no trace of centrosphere 

 or aster at the anaphase or beside the resting nucleus. There are eight 

 chromosomes in the gametophyte, sixteen in the sporophyte. 



In reply to doubts as to the correctness of previous observations, 

 Prof. J. B. Farmer f reiterates his statements of the presence of a 

 quadripolar spindle, and of the existence of centrospheres in the divi- 

 •sions of the nucleus in the spore mother-cells of Pellia epiphylla. 



* 



Ann. of Bot., xv. (1901) pp. H7-80 (2 pis.). Cf. this Journal, 1894, p. 591. 

 t Tom. cit., pp. 431-3. Cf. this Journal, 1896, p. 90. 



