AUTUMNAL CHANGES IN LEAVES OP BVBEGEBENS. 299 



grated and impart a brown color to the [jrotoplasmic mass of the 

 cells ; but in the leaves of the second tj'pe the color is due to 

 a highl}- refractive reddish or yellow mass (supposed to be tan^ 

 nin), concealing from a surface view the clustered chlorophyll 

 granules within, which retain their vivid hue. In aU cases of 

 evergreen leaves the granules of chlorophyll, at the beginning 

 of the cold season, pass from the walls to the centre of the cells, 

 and are there' aggregated in compact clusters. Their normal 

 condition is restored in the warm days of early spring. 



797. Kraus has examined the changes in autumn in the chloro- 

 phyll of Rnscus aculeatus. He finds that in this plant some of the 

 more superficial cells under the epidermis contain minute granular 

 masses of a brownish color, but no chlorophyll granules are to 

 be distinctly seen, and that the subjacent cells have more or less 

 broken-down granules which are yellowish or brownish green. 

 In the cells making up the more spongy tissues there are a few 

 chlorophyll granules quite intact, but there are indications that 

 some others have been completely destroyed and their coloring- 

 matter taken up by the surrounding protoplasm, apparently in a 

 state of solution. 



798. It was thought bj' Kraus that the winter change in the 

 character of the chloroph}!! was due to the lower temperature. 

 He based his views largely upon experiments with a branch of 

 Buxus (Box) ; but it has been shown by Batalin^ and Askenasy ^ 

 that light has a more important influence upon the chlorophj'll 

 than changes of temperature. 



799. The raw materials required for assimilation, and their 

 reception by the assimilating organs. These are (1) water and 

 (2) carbonic acid. In earlier chapters it has been shown in 

 what manner and to what extent water and small traces of min- 

 eral matters are brought from the soil into the plant. It is 

 now necessarj' to ascertain in what way carbonic acid enters 

 the organism and is appropriated by it. 



800. Absorption of carbonic acid by water plants. These can 

 absorb carbonic acid substantially as they absorb mineral salts, 

 directly from the water in which they live. The amount of car- 

 bonic acid found in rain and other waters is variable, ranging, 

 according to the best authorities, from about one per cent to 

 considerablj' less than one tenth of one per cent. The amount 

 existing in the free state in natural waters in which plants thrive 



1 Botanische Zeitung, 1874. 



2 Botanische Zeituns, 1875. 



