GOLOUKS OF PLANTS. 393 



and Calathea zebrina, where there are yellowish spots ; or in many 

 Arums, where the spots are of a red colour. The whitish or brown spots 

 which occur on leaves are often produced by thickened cells contain- 

 ing peculiar colouring matter, underlying the chlorophyll cells. In 

 such cases variegation might be traced to an alteration in the epider- 

 mal cells ; and the same is true of certain bright colours assumed by 

 the surfaces of some leaves. The juices of many plants are colourless 

 when contained in the vessels, but become milky or coloured by 

 exposure to the air. Thus, the ' sap of (Enanthe crocata becomes > 

 yellow, that of Ohelidonium becomes orange, that of Madder changes 

 from yellow to red, and that of some Boletuses becomes blue or bluish- 

 green. In some instances the changes have been prevented by keeping 

 the cut or broken surfaces in nitrogen, or hydrogen, or carbonic acid, and 

 thus preventing their exposure to oxygen. It is said, however, that the 

 change of colour in the Madder does not take place in pure oxygen. 



The bark, at first green, becomes often of a brown colour from 

 the thickening of the cell-walls,' as well as the deposition of brown 

 matter. Similar changes take place in the woody tissue, giving rise 

 to the coloured duramen of many trees, as the Laburnum, Guaiac, 

 Ebony, etc. Such changes, however, depend on chemical actions 

 going on in the interior of stems, and are not due to the direct 

 influence of the air. The colour of wood, however, is generally 

 deepened when exposed to the atmosphere. 



The red, blue, and yeUow colours pf flowers depend on fluid or 

 semifluid matters contained in superficial cells, which can be detached 

 with the cuticle. In petals, different cells frequently contain different 

 kinds of colouring matter, thus giving rise to variegation. By the 

 juxtaposition and mechanical mixture of various cells different tints 

 are produced ; and the colours are also modified by the nature of the 

 cuticle through which they are seen. In the interior of petals the 

 colour is generally more or less yellow, but it is modified when seen 

 through superficial cells. Along with the colouring matter there is a 

 colourless substance present, the relative quantity of which varies, 

 and hence the colour may be deeper or fainter. In flowers, as weU as 

 in leaves, the colours appear to depend on the action of light. It has 

 been said, however, that the powerful action of solar light, in some 

 cases, tends to decolorise flowers. Hence, tulips are screened by 

 floriculturists from the direct rays of the sun. The leaves of herbar 

 ceous plants also, when exposed to the direct rays of the sun, do not 

 acquire so deep a green as when they are subjected merely to a bright 

 daylight. 



The colours of flowers have been arranged in two series : — 1st, 

 The xanthic (^avSos), yellow; and 2d, The cyanic (xuavoe), blue; 

 and it has been shown that plants in general may be referred to 

 one or other of these series, while red is common to both series, 



