MINUTE STRUCTURE OF PLANT HYBRIDS. 279 



state.""" In certain rare cases, e.g., endosperm cells, proliferation of the nucleolus may 

 produce a temporary multi-nucleolar state, while the nucleus and cell can divide at a later 

 period. Behaviour of the endo-nucleolus and nucleolus during division causes me to regard 

 these as the special cell-centres, and this is well illustrated in species of Spirogyra, where 

 the whole role of nuclear division is subsidiary to the nucleolus, and is only initiated subse- 

 quent to indications of commencing division in it.t We regard this as the true explanation 

 of division processes in other plant cells. Now in Spirogyra one can readily see that the 

 nucleolar material not only forms the main mass of chromatic substance, but that it is 

 connected by an extremely fine network system with the nuclear membrane, which is also 

 chromatic, and during division breaks down to fuse with the radiating threads from the 

 nucleolus. In re-formation of the daughter nuclei also round the daughter nucleoli, the 

 nuclear membrane gradually reappears, first on the outer poles or nuclear faces, but some 

 of the nucleolar threads can be traced to radiate out through and beyond the nuclear 

 membrane and across the cell-cavity to the pyrenoid centres. Now, in my earlier studies of 

 Spirogyra^ I was puzzled to understand how these radiating threads that were originally 

 continuous in the nuclear spindle seemed to separate as deposition of the cell-septum took 

 place between. Recent careful study with high powers reveals that from the pyrenoid 

 centres of some bands extremely fine chromatic threads stretch across to, and connect, 

 the pyrenoid centres of other bands. A connection of these from one ceil to another has 

 not as yet been traced, but, apart from observations on other plant-cells which favour 

 it, the strong probability is that such exists, for this network is quite continuous 

 during division up to time of deposition of the cell-partition, and as the latter is 

 laid down by union of micellae from the peripheral protoplasm and from the spindle 

 threads, these may retain delicate continuations of their substance through the formed 

 partition. 



We would consider, then, that the nucleolus is the special chromatic and cell centre ; 

 that it sends out fine radiating processes — the intranuclear network — which partially 

 fuse externally to constitute the nuclear membrane, the interspaces of the network being 

 occupied by nucleoplasm concerned in metabolic change ; that radiating continuations of 

 the chromatic substance pass out beyond the nuclear membrane, and form a network in 

 the protoplasm, while we would suggest for future proof or disproof that they further 

 may be continued through wall pores to form an intercellular chromatic connection. 



Not only in Spirogyra but in leaf cells of Dioncea and of Masdevallia radiating 

 chromatic threads have been traced. 



The question now arises as to the nature and origin of the chromatic substance. 

 This is pretty generally viewed now by biologists as sexual substance par-excellence, and 

 as being the bearer of hereditary characters. To explain its distribution in each cell, we 

 may consider with most biologists that the simplest plant and animal cells have no 



* Trans. Roy. Soc. Edin., vol. xxx., 1881-82. 



+ Confirmatory observations on two species of Spirogrjra will shortly be published, giving details. 



X Trans. Bot. Soc. Edin., vol. xiv., 1882. 



