ELEMBNTAEY STRUCTURE. 27 



the nucleolus are best observed under the action of iodine, -which 

 colours them yellow or brownish. The size of the nucleus in 

 proportion to the cavity of the cell varies greatly ; thus in 

 a very young cell of newly-formed parts the nucleus occupies 

 nearly, or entirely, the whole cavity ; while in the cells of other 

 parts it is of but small size in proportion to the cavity, ^ls.. 

 cells increase i n a^e the nuclei comjnonly disap pear ; bnt in sottip! 

 cases the V I'gmai n as long as the cel ls retai n their vitality : while 

 in othersThey appear to be conver te^^ into chlorophyl l or starc h 

 ^anule's. "" ~ _———___ 



' rt will afterwards be shown, when treating of cell-develop- 

 ment, that the substances just described under the names of 

 Protoplasm, Primordial Utricle, and Nucleus, and which are 

 especially present in very young cells, are actively concerned in 

 the origin and development of new cells. As the cells increase 

 in age, and when mature, a great variety of other substances are 

 found in them, which have been either formed in the plant; 

 or have been obtained from the soil, or substance upon, or in 

 which the plant grows. Th ey are all dissolved, or float in a 

 watery liqui dj^whijch_is^onimon ly called sap l ~~ 



jb^cr^.— ThlsTiquid may be first ohserved in small vacuoles or 

 cavities existing in the protoplasm, and it then constitutes but a 

 small portion of the cell contents ; but as the cells become ma- 

 ture, it continues to increase in quantity until it ultimately fills 

 their cavities. The amount of sap will necessarily vary accord- 

 ing to the conditions under which the plant is placed, especially 

 as regards the amount of water it can absorb or exhale ; but 

 a s a rule, ^ with but few excepti ons, it cannot altogether_ dis- 

 appear from the cell»~or"t^' different' organs of the higher 

 illiouL^'PsirQying^the rife_ofthose~organT! J Iauy of the 

 ~^'^'~~ )ecome completely dried 



but will still retain their 



ine sap is~in~rare "cases coloured by substances which are dis- 

 solved in it, but commonly it is colourless, and resembles com- 

 mon water. It contains, as we have seen, various substances 

 dissolved or floating in it. Three of these we shall now describe, 

 namely, ChJorophyl i, Starch, a nd Baphides. 



ChloropliyU.- — This name is applied to the green colouring 

 matter of plants. It is especially abundant in the cells which 

 are situated just beneath the surface of leaves. It occurs either 

 as an amorphous substance, or far more generally xmder the 

 form of granules, which float in the sap, or are more or less 

 adherent to the wall of the cell {fig. 62). 



The nature of chlorophyll granules is by no means well ascer- 

 tained. Some observers describe them as consisting of soft mu- 

 cilaginous solid matters ; while others define them as small 

 vesicles containing a green liquid. When they are acted upon 

 by alcohol or ether they retain their former size, but lose their 



