NUCLEUS. 



[ 548 ] 



NUCLEUS. 



altogetlier unknown. The nucleus may be 

 observed most easily in the pareuebymatous 

 cells of the herbaceous structures and flowers 

 of Monocotyledons (PI. 46. fig. 28/>), or in 

 the young cells of the hairs of Flowering 

 plants generally (PI. 47. figs. 8, 9b), or in the 

 embryo-sacs of ovules (PI. 47. tigs. 4-6) ; 

 in such cases the characters are well defined 

 and luimistakable. It consists of a lenti- 

 cular body formed of more or less granular 

 protoplasm, with one or more well- or ill- 

 defined bright points or cavities (nucleoli) in 

 the interior. Wherever it appears throughout 

 the higher plants, it seems to possess the 

 same characters ; but it ma}' be absent in 

 the cells of many Algse, Lichens, and 

 Fungi. 



( )rilinarily nuclei are found attached to 

 the side of cells, or forming the centre of 

 radiating protoplasmic filaments (PI. 47. 

 fig. 1)) : sometimes, however, the nucleus is 

 suspended in the middle of the cavity of 

 the cell by filamentous processes of proto- 

 plasm ; in all such cases it forms a kind of 

 centre for the circulation of the protoplasm 

 where this exhibits movement (Rotation), 

 find it is itself carried about to a certain 

 extent by the currents. 



The nucleoli (PI. 47. fig. 8 ») of the.se 

 larger nuclei are apparently usually more or 

 less 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, occupying to the entire cell-contents 

 the same relation as the nucleoli to large 

 nuclei, for example, in the spores of Lichens 

 (PI. 37. fig. 7), Fungi, itc. In the lower 

 Confervoid Algse the nucleus (or nucleolus) 

 appears to be represented by the entire cell- 

 contents (PI. 7), in which one or more well- 

 defined granules often occur, representing 

 nucleoli ; in certain stages, however, a larger 

 granule is met with, coloiu'ed by chloro- 

 phyll, which some regard as a nuclevis ; 

 this disappears totally at particulnr epochs, 

 and is replaced by starch-granules or oil- 

 globules. The bright-coloured point, or 

 " eye-spot," seen very generally in the Zoo- 

 SPOEES 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, granular, or lenticular 

 masses, which become gradually defined in 

 the substance of a collection of protoplasm 

 accumulated at the upper end of the cell 

 (PI. 47. tigs. 1-4). This is a spontaneous 

 isolation of a portion of the protojilasm to 

 become the foundation of a new cell. We 

 may compare this with the segmentation of 

 the entire mass of contents of the cells of 

 Confervae in the formation of Zoospobes, 

 which may perhaps be regarded as at 

 first free nuclei. In cells multiplying by 

 division, a division of existing nuclei has 

 been observed to take place in certain cases, 

 as in the hairs of Traclcscantia (PI. 47. figs. 

 8 & 9) ; but in other .similar cases of division 

 no nuclei are observed (PI. 47. tigs. 10 k 11). 

 In Tradcscan/iUfihe oval parent nucleus fills 

 up the end of the growing cell, so that the 

 division of tlie nucleus is almost synonymous 

 with the division of the primordial utricle. 

 But in this case, as in the development oi 

 cells from free nuclei, as indicated in 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 Zi/f/nema, a 

 division of the free suspended nucleus pre- 

 cedes the division of the large primordial 

 utricle. 



Mohl describes a division of nuclei as 

 occurring 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; 

 but 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 pi-e- 

 sent, may be divided with the cells. Others 

 imagine that they are merely the original 

 " mould " of protoplasm on which the cel- 

 lulose membrane of the nascent cell is de- 

 posited, and which is left unaltered when 

 this expands (the phenomena in Spirogyra 

 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. 



They are best seen in very young cells in 

 all cases ; in nascent tissues they almost or 

 quite fill the cavit\" of the young cells. As 

 the cells grow older, their histoiy difl^ers in 

 dillerent cases. Sometimes they persist 



