20 ORGANOGRAPHY. 



the form of granules or globules, which float in the cell- 

 sap, or are more or less adherent to the walls of the cell 

 (fg- 46). 



The nature of chlorophyll is by no means well ascertained. 

 Some observers describe the granules as consisting of soft 

 mucilaginous solid matters ; while others describe them as small 

 vesicles containing a green liquid. When they are acted upon 

 by alcohol or ether they retain their form and size but lose their 

 green colour, hence it is clear that they consist of a substance 

 which is coloured green by the presence of colouring matter 

 diffused through them which is soluble in alcohol or ether. The 

 granules wlicn thus freed from green colouring matter are 

 coloured yellow by iodine, and therefore contain nitrogen. It 

 would seem probable from this, as well as from their common 

 occurrence around the starch granules (which, as we shall find 

 in speaking of starch, are developed from protoplasm), that 

 they are simply granules of protoplasm coloured by a green 

 colouring matter to which the name of chlorophyll properly 

 applies. This chlorophyll or green colouring matter is a com- 

 plex substance, consisting, according to Mulder, of wax, and 

 a matter resembling indigo. An incomplete analysis gives its 

 composition Cig Hjr No Og. Chlorophyll is only formed under 

 the influence of hght, it never occurs therefore in structures 

 removed from that agent, but exclusively on the parts of plants 

 near the surface. In the autumn it undergoes certain changes 

 which are not well understood, by which it loses its green 

 colour, and assumes various shades of red or yellow. 



All the colouring matters contained in the cells which are not 

 green, and to which the peculiar tints of the petals and other 

 parts are due, are frequently comprised under the common 

 name of chromule. These are of various natures, and will be 

 treated of hereafter when speaking of the cause of colour in 

 plants. 



Starch. — There is no substance contained in the cells which 



has given rise to more discussion as to its origin and nature than 



Fig. 36. starch. It is, with the exception of protoplasm, 



_^ * the most abundant and universally distributed of 



all tlie cell-contents, occurring as it does, more 



or less, in all parenchymatous cells (Jig. 36), 



except those of the epidermis. It is, however, 



most abundant in the matured structures of a 



plant, as in tlie pith of steins, seeds, roots, and 



other internal and subterranean organs which 



''the Potatifcon-^ ^^^ rcmovcd from the influence of light. In 



taining starch- thcsc rcs])ects it jircscuts a marked contrast to 



granule*. chlorophyll, whicli occurs only in young and 



vitally active structures, which are placed near the surface of 



