1887-88.] Colouring Matters of Leaves and Flowers. 299 



the starch grains on, or in, the colourless corpuscle (leuko- 

 plasts) or chromoplasts from which the chloroplast arises." 



But, in tlie vast majority of cases, although we may not, 

 as here, have direct evidence of the destructive metabolism 

 of the cell-contents, yet we are fairly entitled to judge 

 from the circumstances of the cell's growth, whether the 

 conditions are those favouring a destructive or a constructive 

 metabolism (anabolism). 



If we consider, in the first place, the conditions accom- 

 panying the most universal of all vegetable colour-changes, 

 that of autumn ; if we associate this colour-change with 

 that of spring (in both these cases the colour appearing not 

 only in trees, but in the smallest herbs, in flowering or 

 fiowerless plants) ; and, finally, if we associate with these two 

 colour phenomena, that of reproduction, we may see at once 

 an identity in the conditions which enable us to bring 

 them all under a homologous explanation. 



Colour " among young shoots of spring " is where carbon- 

 assimilation has not yet commenced, but where the growth is 

 at the expense of stores obtained in the previous season ; 

 colour in autumn — in the brilliant rosettes of Seduni or 

 Vaccinium, in Azalea or Acer (which are as attractive to 

 insects as any flower) — is where assimilation has ceased, 

 and further elaboration or alteration of the cell-contents 

 takes place ; colour in the cells specialised for reproduction 

 is also essentially produced where assimilation has given 

 place to an opposite function, by which the young ova 

 grow at the expense of materials elsewhere assimilated by 

 the parent. 



To give two excellent examples of this, we may notice 

 the colour of the flowers of Cuprcs^us Lawsoniana (seen at 

 our last meeting), or the change of colour in mosses and 

 many algae when reproduction commences. 



In regard to the formation of flowers among Phanerogams 

 — if the nutrition be very abundant under conditions 

 favouring carbon-assimilation — gTcen and healthy leaves will 

 be produced, as is in the experience of every gardener, often 

 to the exclusion of flowers. 



Professor A. Sachs (who was conducting a series of experi- 

 ments with the view of determining the cause of flower-pro- 

 duction) noticed that when plants were grown in yellow light 



