464 BOTANY OF THE LIVING PLANT 



of its continued nutrition. How this has worked out in detail in 

 Mosses and Ferns, and ultimately in the Seed-Plants, will be dis- 

 cussed in Chapter XXXII. Here it must suffice to remark that the 

 subsequent changes seen in such Plants have to do with the details 

 of transmission and of nursing of the gametes. They do not involve 

 any further distinction of the male and female gametes, in respect of 

 size or structure, than that already established among the Higher 

 Thallophyta. 



Functions of Sex. 



To form an opinion on the function of the gametes in sexual propa- 

 gation, a knowledge of their structure is necessary ; also of the rela- 

 tion of their structure to that of other cells of the plant-body. The 

 nucleus, w^hich takes a prominent part in the process of fertilisation, 

 and itself constitutes almost the whole of many spermatozoids, will 

 specially claim attention. Moreover, the facts already recognised 

 in Chapter II., p. 22, — that every nucleus is derived from a pre- 

 existing nucleus, and that in ordinary cell-division the parent nucleus 

 is partitioned into two exactly corresponding halves, — suggest that 

 the nucleus has a special relation to the facts of heredity. The detail 

 of the process of division of the nucleus of an ordinary somatic cell 

 will then be the natural starting point for the further discussion of 

 Sex and Heredity. 



Somatic Division. 



The resting nucleus of a vegetative cell has a reticulate structure. The net- 

 work is recognised chiefly by the punctated appearance of the anastomosing 

 threads. These threads consist of a substance called Hnin, which bears minute 

 granules of chromatin, so called because they take up certain stains more readily 

 than the rest of the nucleus. One or more nucleoli, composed probably of 

 reserve material, are closely connected with the network, while the interstices 

 are filled with nuclear sap, and the whole is delimited by a nuclear membrane 

 (Fig. 392, i.). WTien such a nucleus is about to divide, the network is drawn 

 together at a certain number of points, round which bodies, at first irregular 

 but ultimately of filamentous form, are segregated. They take a deep stain, 

 and are accordingly called chromosomes. It is probable that they maintain 

 their identity, though in a diffused form, during the resting period of the 

 nucleus, but become evident by their condensation as division approaches 

 (Fig. 392, ii.) . They group themselves in an equatorial plane, which corresponds 

 to the plane of division of the cell. Thus placed they constitute what has 

 been called the nuclear, or equatorial plate (Fig. 392, iii.). Each such chromo- 

 some splits longitudinally, and the two equivalent halves separate and move 

 apart, the one to form a constituent of one daughter nucleus, the other of the 

 other (iii. iv.). The mechanism by which this separation is carried out is by 



