140 A TEXT-BOOK OF BOTANY. 



to the studies of the protoplasmic structures of the cell, especially 

 the nucleus; the reason for this being that all of the vital phe- 

 nomena of which living organisms are capable have their origin 

 in these substances. The nucleus is regarded as a controlling 

 center of cell activity, for upon it all growth and development of 

 the cell depend, and it is the agent for the transmission of specific 

 qualities from one generation to another. Furthermore, cytolo- 

 gists look upon the chromatin material of the nucleus as being 

 the agent for the transmission of individual characters to offspring. 

 The reason for this is that in the male generative cell it is prac- 

 tically only the nucleus which fuses with the egg-cell, no other 

 substances entering into the union. The centrosomes are usually 

 apparent during the process of nuclear division and by some 

 are regarded as the controlling organ of cell division, hence they 

 are known as the dynamic centers of the cell. The functions of 

 the plastids and cytoplasms are largely, if not entirely, connected 

 with the synthesis, transportation, and dissociation of metabolic 

 substances. 



NON-PROTOPLASMIC CELL-CONTENTS. 



The non-protoplasmic constituents of plants may be said to 

 differ from the protoplasmic cell-contents in two important partic- 

 ulars, namely, structure and function. For convenience in con- 

 sidering them here, they may be grouped as follows : 



(1) Those of definite form including (a) those which are 

 colloidal or crystalloidal, as starch and inulin ; (b) those which 

 are crystalline, as the sugars, alkaloids, glucosides, calcium oxa- 

 late ; (c) composite bodies, as aleurone grains, which are made 

 up of a number of different substances. 



(2) Those of more or less indefinite form, including tannin, 

 gums and mucilages, fixed and volatile oils, resins, gum-resins, 

 oleo-resins, balsams, caoutchouc, and also silica and calcium car- 

 bonate. 



I. SUBSTANCES DEFINITE IN FORM. 

 COLLOIDAL OR CRYSTALLOIDAL. 



Starch is the first visible product of photosynthesis, although 

 it is probable that simpler intermediate products are first formed. 

 This substance is formed in the chloroplastid (see Frontispiece) 

 and is known as ASSIMILATION STARCH. Starch grains are 



