5 6o BOTANY OF THE LIVING PLANT 



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

 and Ferns, and ultimately in the Seed-Plants, has been discussed in 

 Chapter XXXIV. Here it must suffice to remark that the sub- 

 sequent 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. 



The Nucleus and Heredity. 



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

 pagation, a knowledge of their structure is necessary ; also of the re- 

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

 nucleus, which 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. 20 — 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 nuclear division will then be the natural starting point 

 for the further discussion of Heredity and Variation. 



NUCLEAR DIVISION 



The Resting Nucleus. 



The " resting nucleus " of a vegetative cell — that is the nucleus when 

 not in a state of division — has the appearance of a meshwork (nuclear 

 reticulum) of fine threads embedded in a clear ground-substance 

 (nuclear sap) and delimited from the cytoplasm by an exceedingly 

 delicate nuclear membrane (Fig. 428, a). The stains commonly used 

 in cytological studies do not affect the whole of the meshwork to the 

 same extent, certain parts becoming more heavily stained than the 

 rest. It has therefore long been customary to describe the nuclear 

 reticulum as composed of faintly staining linin with localised masses 

 of deeply-staining chromatin. But there may be doubt as to the 

 morphological distinctness of these two components. Enclosed in the 

 nucleus are one or more denser, rounded, heavily staining bodies, the 

 nucleoli. 



