CHANGES AFTER THE FRUIT HAS RIPENED. 143 



§ 5. Of the chemical changes which take place after the ripening of 

 the fruit and seed. 



When the seed is fully ripe, the functions of annual plants are dis- 

 charged. They no longer reciuire to absorb and decompose carbonic 

 acid, for their growth is at an end. Their leaves begin, therefore, to 

 take in oxygen only, become yellow, and prepare along with the entire 

 plant, for being finally resolved again into those more elementary sub- 

 stances from which they were originally compounded. 



On trees and perennial plants, however, a further labour is imposed. 

 In the ripened seed they have deposited a supply of food sufficient to 

 sustain the germ thai may spring from it, until it is able to seek food for 

 itself; but the young buds already formed, — and which are to shoot out 

 from the stem and branches in the ensuing spring, — are in reality so 

 many young plants for which a store of food has yet to be laid up in the 

 inner bark, and in the wood of the tree or shrub itself. 



In the autumn, the sap of trees and permanent shrubs continues to 

 flow rapidly till the leaf withers and falls, and the food of the plant is 

 converted partly into woody fibre, as was the case during the earlier 

 period of the year, and partly into starch. The former is deposited be- 

 neath the inner bark to form the new layer of wood by which the tree is 

 annually enlarged ; the latter — partly in the same locality, as in the 

 birch and pine — partly throughout the substance of the wood itself, as in 

 the willow — while in the palrn trees and cycadeae, it is intermingled 

 with the central pith. The chemical changes by which the food is ca- 

 pable of being converted into these substances have already been con- 

 sidered. They proceed during the entire autumn, do not cease so long 

 as the sap continues to move, and even in the depth of winter slowly and 

 silently operate in storing up farinaceous matter — in readiness, like the 

 starch*^in the seed, to minister to the nourishment of the young bud, when 

 the warmth of the coming spring shall awaken it from its long sleep. 



§ 6. Of the rapidity with lohich these changes take place^ and the 

 circumstances by which they are promoted. 



But remarkable as those chemical changes are, the rapidity with 

 which they sometimes take place is no less surprising. 



From carbonic acid and water we have seen that the plant, by very 

 intelligible processes, can extract the elements of which its most bulky 

 parts consist — andean build them up in many varied ways, most of which 

 are probably beyond the reach of imitation. But who can understand or 

 explain the extraordinary activity which pervades the entire vascular 

 system of the plant, when circumstances are favourable to its growth? 



A stalk of wheat has been observed to shoot up three inches in as 

 many days, of barley six inches in the same time, and a vine twig 

 almost two feet, or eight inches a day (Du Hamel). Cucumbers have 

 been known to acquire a length of twenty-four inches in six days, and 

 in the Botanic Garden at Brussels I was shown a bamboo five inches in 

 diameter, which had increased in height nine feet in twenty-seven days, 

 sometimes making a progress of six to eight inches in a day. In our 

 climate we meet with few illustrations of the rapidity with which plants 

 are capable of springing up in the most favourable circumstances, and 

 the above examples probably give us only an imperfect idea of the ve- 



