i6 



%ORPIIOLOGr OF THE CELL. 



B. Cell-formation in Growing Organs: Vegetative Cell-formation. 



This mode of cell-formation consists almost invariably in the bipartition of a mother- 

 cell ; there is no or scarcely any perceptible rounding off or contraction of the dividing 

 protoplasm ; the two daughter-cells entirely occupy the place of the mother-cell. 



I. The partition-^ all is formed gradually , The protoplasm of the mother-cell projects 

 in the plane of division as a circular protuberance, and a ridge of cellulose is formed 

 in it which finally developes into a complete septum. 



A clear and well-studied example 

 ft is afforded in the stouter species of 



the g^^wAS Spirogyra'^ . In order to 

 observe the divisions, it is necessary 

 to place filaments that are in active 

 growth in very dilute alcohol after 

 midnight, that they may be exam- 

 ined by day, the divisions taking 

 place only by night. Fig. 13 A 

 shows a living cell of a filament of 

 5. longata by day \ B io E the stages 

 of division at night ; the protoplasm 

 of the cells is contracted by the 

 alcohol. 



[The phenomena of division fall 

 into two groups, namely those which 

 attend the division of the nucleus, 

 and those which are connected with 

 the ingrowth of the partition-wall. 

 The nucleus of a cell which is about 

 to divide becomes broader, assum- 

 ing the form of a biconcave lens, 

 and its nucleolus breaks up into 

 irregular granules, which, together 

 with its other granular contents, 

 begin to form a nuclear disc in the 

 equatorial plane. A delicate stria- 

 tion is now apparent in what is 

 becoming the long axis of the nu- 

 cleus, at right angles to the nuclear 

 disc, and a characteristic nuclear spindle is gradually produced. The nuclear disc splits 

 into two halves lying side by side, each of which travels to the corresponding pole of 

 the nucleus ; thus two new nuclei are constituted which are connected by fibrillae. 



The first indication of the formation of the partition- wall is the accumulation of 

 protoplasm at about the middle of the length of the cell, so as to form a slight annular 

 protuberance into the sap-cavity: this takes place about the same time as the first 

 changes in the nucleus. Within this protuberance a rim of cellulose is formed which 

 grows inwards, carrying before it the spiral chlorophyll-band (Fig. 13, 5 and C) ; 

 soon the chlorophyll-band is cut through, and margins of the ring of protoplasm 

 coalesce in the middle so as to form a plate, to which the fibrillae connecting the two 

 nuclei, which have been formed in the meantime, become attached ; in this plate the 

 remainder of the cellulose wall is formed. The nucleus of each cell now travels to 



Fig. ^-^.—Spirogyra longata (X 550). A a cell in the living state; 

 B, C cells laid in dilute alcohol during the division by night ; D, E central 

 portion of cells in the act of division. 



^ This case was the first of all the processes of cell-formation that was accurately examined : 

 H. von Mohl first described it in 1835 '^^ Conferva glomerata. Mohl, Vermischte Schriften bot. 

 Inhalts, Tubingen 1845 : [Strasburger, Zellbildung und Zelltheilung, 1880.] 



