I20 PLANT LIFE. 



delta at its mouth.^ No two plants, however, have exactly 

 the same method of root formation. Thus, amongst our 

 common trees a long main root is characteristic of the 

 Oak, Ash, Edible Chestnut, and Lime tree, whilst others 

 have very wide horizontally spreading root branches, 

 like the Spruce, Birch, and probably the Douglas Fir, and 

 are therefore more liable to be uprooted by the wind. 



Besides this power of growing in a definite direction, 

 roots are very sensitive to both moisture and light. 

 They seek out moist places in the soil and bury 

 themselves in the dark so far as possible. During 

 their exploration in the soil their tips are protected by 

 little thimbles of dead tissue, the root-caps, which save 

 the delicate growing tissue from accidental injury. 

 They can push small obstacles aside ; if they meet a 

 stone which they cannot thrust away, a message is sent 

 back to the growing part ; this latter begins to curve to 

 one side, and the young root will, probably, circum- 

 vent the stone, and proceed upon its way. Only 

 the extreme tip of the root is thus sensitive. If 

 in the course of thus bending aside, the growing part, 

 not the tip, is brought against the stone then the 

 reverse happens. The growing part bends round 

 the obstacle and presses itself against it. These two 

 properties supplement and assist one another, so that a 

 root can find its way down any irregular passage, or 

 crevice. 



It has already been stated that, in the case of 

 common trees, every species has the desire to form its 

 root system in a particular manner ; but this is often 



^ The same comparison, reversed, applies to the supply of food material 

 by the sieve tul:)es. The leaves are the laboratories, and their supply is 

 collected into the phloem of the stem and eventually distributed, in a 

 delta-like manner, to the rootlets. 



