BOTANICAL CYTOLOGY. As yet little is known regarding the structure of protoplasm in plants. hut the which have been made favor the assumption that its structure is iden tical with that in animals. Much more attention has been paid to the nucleus. Even the small nuclei of many of the algae and fungi have been studied, and the details of their structure and mode of division quite accurately determined. No organ of the cell has been so assiduously in vestigated as the chromosome, but nevertheless most of its important problems remain to be solved. The fact that the number of chromo somes is constant for a given species, and the phenomena of fertilization indicate that the chromosome is a permanent organ of the cell, but its life history from one cell generation to another has not yet been traced, the identity of the several chromosomes being lost in the resting nucleus. In the flowering plants the splitting of the chromosomes during nuclear division is generally conceded to be longitudinal in all cells except spore-producing cells in which the reduc tion of the chromosomes is taking place, and even here most botanists believe that the splitting is longitudinal. although a transverse splitting, i.e. a reducing division in the sense of the Weiss man school, has been reported by investigators of undoubted ability. Both observations and theories are still very conflicting. The origin and development of the achromatic figure have re ceived large attention, especially during the past five or six years. It was formerly supposed that the achromatic figure always rose under the influence of the centrosomes, but recent observa tions have made it very doubtful whether a centrosome exists at all in the angiosperms, and it is almost equally doubtful whether such an organ exists in the gymnosperms and pterido phytes, unless the 'blepharoplast,' a eentrosome like body which develops the cilia of the male cell, be interpreted as a genuine centrosome. In the other groups, except the mosses, which have received scant attention, an undoubted centro some has been demonstrated.
The development of the sex cells, from the earliest appearance of the archesporium up to the time of fertilization, has been repeatedly studied in various plants, but the work has been mor phological rather than cytological, little atten tion having been paid to the details of cell con tents except in case of mother cells. In the
study of cells more immediately concerned in fertilization, the cells of the sporogenous tissue have been slighted. Some of the most impor tant cytological work deals with the problems of fertilization. The question of sexuality in the Ascomycetes has received a definite answer in the case of several forms by the demonstration of an actual process of fertilization. The fusion of the sex nuclei in ferns has been described with more or less completeness. In the gymno sperms, where the sex cells are extremely large, the process of fertilization has been more satis factorily investigated, and it has been found that both the nucleus and the cytoplasm of the male cell enter the egg, but that the nucleus slips out from its cytoplasmic mantle before it reaches the nucleus of the egg. The male nucleus with its nuclear membrane still intact is then received bodily into the much larger egg nucleus. The chromatin of the two nuclei in the form of two distinct spirems has been observed, and it has been suggested that the chromatins Of the two nuclei may remain distinct during the later stages of fertilization. and even during the cell divisions which follow. In the angiosperms, while the union of the sex nuclei has been re peatedly observed, the behavior of the chromatin is practically unknown. Two male cells are dis charged from the pollen-tube into the embryo sac; the nucleus of one of these cells unites with the nucleus of the egg and the first cell of the sporophyte is formed. It has recently been found that the second male cell often unites with the definite nucleus of the embryo-sac formed by the fusion of the two polar nuclei, so that there is a 'double fertilization.' Double fertilization has been observed in monocotyledons and dico tyledons, but whether it is the usual method of fertilization is not entirely settled. While it is becoming conceded that the problems of heredity must be ultimately problems of the cell, nearly all the work of botanists along this line must be classed as morphological. See CELL (in plants) ; EMBRYOLOGY SEX,