LECTURE VII. 



Chemical changes by which the substances of which plants chiefly consist are formed from 

 those on which they live.— Changes during germination— during the growth of the plant— 

 during the ripening of fruit. — Autumnal changes. 



Having thus considered the nature and chemical constitution of those 

 substances which constitute by far the largest part of the solids and 

 fluids of living vegetables, we are now prepared for the further question 

 — hy what chemical changes these substances of which plants consist, are 

 formed out of those on which they live ? 



The growth of a plant from the germination of the seed in spring till 

 the fall of the leaf in autumn, or the return of the succeeding spring- 

 lime, may in perennial plants be divided into four periods — during which 

 they either live on different food, or expend their main strength in the 

 production of different substances. These periods may be distinguished 

 as follows : — 



1°. The period of germination — from the sprouting of the seed to the 

 formation of the perfect leaf and root. 



2°. From the expansion of the first true leaves to the period of flow- 

 ering. 



3°. From the opening of the flower to the ripening of the fruit and 

 seed. 



4°. From the ripening of the seed or fruit, till the fall of the leaf and 

 the subsequent return of spring. On the ripening of the fruit the func- 

 tions of annual plants are in general discharged, and they die ; but per- 

 ennial plants have still important duties to perform in order to pr€^are 

 them for the growth of the following spring. 



The explanation of the chemical changes to which our attention is to 

 be directed will be more clear, and perhaps more simple, if we consider 

 them in relation to these several periods of growth. 



§ 1 . Chemical changes which take j>laee during germination and during 

 the development of the first leaves and roots. 



The general nature of the chemical changes which take place during 

 germination is simple and easy to be comprehended. 



Let us first consider shortly the phenomena which have been observed 

 to accompany germination, and the circumstances wliich are most fa- 

 vourable to its rapid and healthy progress. 



1°. Before a seed will begin to sprout, it must be placed for a time in 

 a sufficiently moist situation. We have already seen how numerous 

 and important are the functions which water performs in reference to 

 vegetable life (Lecture IL, p. 36,) in every stage of a plant's growth. 

 In the seed no circulation can take place — no motion among tlie parti- 

 cles of matter — until water has beer largely imbibed ; nor can the food 

 be conveyed through the growing vessels, unless a constant supply of 

 fluid be afforded to the seed and its infant roots. 



2°.' A certain degree of warmth — a slight elevation of temperature- 

 is also favourable, and in most cases necessary, to germination. • 



