ELEMENTARY STRUCTURE. 19 



obscure, and, according to Henfrey, the term nucleus is here very 

 loosely applied to irregular granular structures Avliich do not 

 represent the nuclei of Flowering Plants. 



The nuclei originate in two ways ; either as new formations 

 by the aggregation of granules of protoplasm, or by the 

 division of existing nuclei into two or more separate ones. 

 (See Cell- development, page 59.) 



As cells increase in age the nuclei either disappear, or persist 

 as long as the cells retain their vitality; or in some cases they 

 appear to be converted into chlorophyll or starch granules. 



The supposed functions of these nuclei Avill be treated of 

 hereafter in speaking of the process of cell-development. 

 Generally we may add that they require further investigation 

 before Ave can pronounce positively on many points con- 

 nected with their stracture and development. 



The substances just described under the names of Protoplasm, 

 Primordial Utricle, and Nucleus, are those Avhich are especially 

 present in very young cells, in which they perform, as will be 

 presently shown, very important functions. As the cells in- 

 crease in age, and when in a mature state, 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 groAvs. It does not 

 enter into our purpose at present to describe all these contents, 

 but only a fcAv of the more important. They are all dissolved, 

 or float in a Avatery liquid, Avhich is commonly called sap. 



Sap. — This Uquid may be first obserA'ed in small vacuoles 

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

 portion of the cell contents; as the cells become mature, the 

 quantity of sap continues to increase, until it ultimately fills 

 their cavities. The amount Avill necessarily vary according 

 to the conditions under Avhich the plant is placed, especially as 

 regards the amount of Avater it can absorb or exhale; but 

 as a rule, with but few exceptions, it cannot altogether disap- 

 pear fi-om the cells of the different organs of the higher plants 

 Avdthout destroying the hfe of those organs. Many of the lower 

 kinds of plants may, hoAvever, become completely dried up, but 

 AviU still retain their vitahty. 



The sap is in rare cases coloured by substances Avhich are dis- 

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

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

 dissolved or floating in it. Three of these we shall noAv de- 

 scribe, namely. Chlorophyll, Starch, and Eaphides. 



Chlorophyll. — This name is applied to the green colouring 

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

 are situated just beneath the surface of leaves. It occurs 

 either as an amorphous substance, or far more generally under 



