NUCLEUS. 



[ 466 ] 



NUCLEUS. 



really hollow, do occasionally occur in the 

 cell-contents. Nageli, who has investigated 

 the subject of nuclei very extensively, states 

 that they exist in every class of plants, and 

 that in those cases where he failed to find 

 them, there w^as a probability of their being 

 concealed in the cell-contents. The nuclei 

 of certain plants exhibit very remarkable 

 peculiarities, especially in Spirogyra and 

 Zygnema. 



Ordinarily, nuclei are found attached to 

 the side of cells, being intimately connected 

 with the Primordial utricle, or, when- 

 ever this is partially absorbed, forming the 

 centre of the radiating protoplasmic filaments 

 this leaves behind (PI. 38. fig. 9); some- 

 times, however, the nucleus is suspended in 

 the cavity of the cell by filamentous pro- 

 cesses of protoplasm; in all such cases it 

 forms a kind of centre for the circulation of 

 the protoplasm, where this exhibits move- 

 ment (Rotation), and it is itself carried 

 about to a certain extent by the currents. 



The nucleoli (PI. 38. fig. 8 n) of these 

 larger nuclei are apparently usually solid 

 granules of a transparent substance, but 

 sometimes they appear more like minute 

 cavities. 



The nuclei and nucleoli of the lower plants 

 are exceedingly obscure ; in a great many 

 cases the so-called nuclei are little different 

 from the nucleoli of the larger forms, occu- 

 pying to the entire cell-contents the same 

 relation as the nucleoli to large nuclei, for 

 example, in the spores of Lichens (PI. 29. 

 fig. 7)j Fungi, &c. Indeed, from our own 

 observations we are led to believe that the 

 term nucleus is very loosely applied in the 

 lower plants, to irregular granular structures 

 which do not represent the nuclei of the 

 Flowering plants, and we doubt the wide 

 diffusion claimed for them by Nageli. In 

 the lower Confervoid Algse the nucleus (or 

 nucleolus) appears to be represented by the 

 entire cell-contents (PI. 3), in which one or 

 more well-defined granules often occur, re- 

 presenting nucleoli ; in certain stages, how- 

 ever, a larger granule is met with, coloured 

 by chlorophyll, which some regard as a 

 nucleus ; this disappears totally at particular 

 epochs, and is replaced by starch-granules 

 or oil-globules. The bright coloured point, 

 or ' eye-spot,' seen veiy generally in the Zoo- 

 spores both of Confervoids and Fucoids, 

 may represent a nucleolus. 



Nuclei originate in two ways. The sim- 

 plest mode is found where they precede free 

 cell-formation, as in the development of the 



germinal vesicles in the embryo-sacs of 

 Flowering plants. Here the nuclei appear 

 first as globular or lenticular masses, which 

 become gradually defined in the substance 

 of a collection of protoplasm accumulated 

 at the upper end of the cell (PI. 38. figs. 1-4). 

 This is a spontaneous isolation of a portion 

 of the protoplasm to become the foundation 

 of a new cell. We may compare this with 

 the segmentation of the entire mass of con- 

 tents of the cells of Confervae in the forma- 

 tion of Zoospores, which may perhaps be 

 regarded as at first free nuclei. In cells 

 multiplying by division, a division of exist- 

 ing nuclei has been observed to take place 

 in certain cases, as in the hairs of Trades- 

 cantia (PI. 38. figs. 8 & 9), but in other 

 similar cases of division no nuclei are ob- 

 served (PI. 38. figs. 10 & 11). In the case 

 of Tradescantia, the oval parent-nucleus 

 fills up the end of the growing cell, so that 

 the division of the nucleus is almost syno- 

 nymous with the division of the primordial 

 utricle. But in this case, as in the develop- 

 ment of cells from free nuclei, as indicated 

 of the germinal vesicles, the cell-membrane 

 in expanding draws away from the nucleus, 

 which remains adherent to or suspended in 

 connexion with a layer of protoplasm lining 

 the cell-wall and forming its primordial 

 utricle. In Spirogyra and Zygnema, a 

 division of the free suspended nucleus pre- 

 cedes the division of the large primordial 

 utricle. 



Mohl describes a division of nuclei as oc- 

 curring in Anthoceros, and most authors 

 who have written on the development of 

 pollen and spores lay great stress on the 

 influence of the nuclei, which they describe; 

 and it is curious that in our own researches 

 we have failed to trace the dependence of 

 the development of the new cells upon nuclei 

 (PI. 38. figs. 10-13). 



The import of nuclei in vegetable cells is 

 certainly still a problem. Some believe 

 they are the universal agents of production 

 of new cells, others that they are not the 

 agents of this in any case, but, when present, 

 may be divided with the cells. Others 

 imagine that they are merely the original 

 " mould" of protoplasm on which the cellu- 

 lose membrane of the nascent-cell is depo- 

 sited, and which is left unaltered when this 

 expands (the phsenomena in Sinrogyra are 

 opposed to this). Some of those who deny 

 their influence in cell-development believe 

 them to be the vital centres of the cells in 

 which they exist. 



