THE BOOK OF NATURE STUDY 



plant down deeper into the soil. This may be observed in lords 

 and ladies, or arum, in the crocus, and in many other plants. It is 

 due to the fact that the oldest part of the root that nearest the 

 stem at a certain age shortens or contracts, owing to changes 

 that take place in the outer cells. These grow shorter in the 



longitudinal direction, and at 

 the same time often increase 

 in width. The apical portion 



FIGS. 15 and 16. Oats sown at 

 different depths. The dotted 

 line marks the surface of the 

 soil. 



of the root has reached a good depth in plants of a certain 

 age, and as the tips of all the branches have become more or 

 less firmly fixed in the soil, the effect of the shortening of 

 the older portion of the root is to pull the whole plant down- 

 wards. A contractile root may be recognised by the trans- 

 verse ridges and furrows which mark it. In the arum the 

 contractile roots arise on the tuber in the late autumn. In the 

 crocus, they are found at the base of the young corn in the spring, 

 and they sometimes even pass through the old corn, situated 

 underneath the young one. Those who have gardens and would 

 like to try experiments on the depth to which bulbs, etc., may be 



