iv LETTERS TO MARCO 25 



and light act as retarding agents. The 

 frosts in winter expand and lighten the soil, 

 raising it up very much as yeast does the 

 dough ; this not only renders it soft and 

 easily pierced by the young plants, but also 

 allows the warmth of the sun to penetrate 

 more thoroughly. Under these conditions 

 the stalks begin to lengthen and push up- 

 wards, whilst the roots serve as anchors 

 below. At the end of exogenous stalks the 

 leaves and flowers grow in a bunchy way, 

 which, offering resistance to the earth, keeps 

 back the end of the stalks and forms the 

 loop. As the stalk keeps on pushing up 

 from below it gradually clears the bunch of 

 leaves from the soil ; it is then that the 

 retarding effect of light begins to tell, for 

 the upper side of the loop being more ex- 

 posed to its influence its growth is checked, 

 whilst the under or concave side, by going 

 on growing, straightens the stalk, and so the 

 young plant is gradually warped as it were 

 into shape. The little drawing of a syca- 



