CYTASE. 720 CYTOLOGY. seeds, cytase is produced at germination to digest the reserve cellulose and render it available as food for the growing embryo. Since the substances which take part in the formation of cell-walls are numerous and diverse, and their composition is still uncer- tain, no adequate kno«ledge has been obtained as to the products of their digestion by cytase. From the cellulose constituents one nr more sugars are produced, possibly by hydrolysis, through de.Ntrins. Cytase acts only on cellulose walls, being unable to attack lignified or cuti- nized walls. It acts most energetically in a weakly acid medium, but is destroyed by tem- peratures of (iO to (io" C. It may be obtained (mixed with diastase) from malt by the method described under Diastase (q.v.). See also Enzymes; and Digestion. CYTHE'RA. See Cerigo. CYTH'ERE'A. See Venus. CYTHE'RIS. A well-known Roman cour- tesan, the mistress of Marcus Antonius and later of Gallus, the elegiac poet. She is referred to under the name of Lycoris in Vergil's tenth Eclogue. CYT'ISUS (Lat., shrubby kind of clover). A genus of plants of the natural order Legumi- nosae, of which some of the species, having long twiggy branches, are popularly called broom, others are called laburnum, while others still are generally known by the name eytisus. The species are numerous — small trees or shrubs, with leaves of three leaflets, and yellow, white, or piirple flowers, natives chiefly of the warmer temperate parts of the Old World. jMany of them are very beautiful, and some are among the esteemed ornaments of our shrubberies, others of our greenhouses. Several species of Cytisus have been recommended as forage plants, stock readily browsing upon their green twigs. Cytisus sco- purius, the Scotch broom, is a form so valued, Avhile Cytisus prolifenis alba, the tagasaste of the Canary Islands, is highly conmiended. Trials of it in California have not substantiated the rather extravagant claims made for it. CYTOL'OGY ( Gk. k&tos, kytos, cell + oyta, logia, account, from Xiyew, legein, to say) . A branch of the sciences of botany and zoology. As histology is largely concerned with tissues, so oytologj' deals principally with cells, the ele- ments which make up the tissues. Although it is only recently that botanists have begun to make a specialtj' of this subject, the work has Iieen prosecuted with great vigor and the subject is beginning to assume some definiteness. The chief problems at present are the structure and activities of protoplasm, the life history of plastids, the structure and function of the nucleus, the reduction of chromosomes, the origin and development of the achromatic figure, the centrosome, the cell-wall, the development of the sex cells, fertilization and the formation of the embryo, and, most difficult of all, the problem of heredity. Botanical Cytology. As yet little is known regarding the structure of protoplasm in plants. but the investigations which have been made favor the assumption that its strxicture is iden- tical with that in animals. 'Much more attention has been paid to the nucleus. Even the small nuclei of manv 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 eliromosome 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 ^'eiss- man school, has been reported by investigators of undoubted ability. Both observations and theories are still very contlicting. The origin and development of the achromatic flgure 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 wliether 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 centrosomo- like bod_y 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 gynmo- 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 obser'ed, and it has been suggested that the chromatins 6f 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- peatedlv observed, the behavior of the chromatin is practically unlcno^^•n. Two male cells are dis-
