28 



THE BOOK OF NATURE STUDY 



buds do not ripen. On the other hand, if the roots are not 

 growing properly the tree is not getting enough food to produce 

 fruit. 



One of the first experimenters on root-pressure was Hales, 

 who carried out a series of investigations on the force of blood- 

 pressure in the arteries. The circulation of the sap through a 

 plant may be compared with the circulation of the blood through 

 the arterial system of an animal. 



ROOT-SYSTEMS. It is interesting to compare the root-systems 



of various plants. 

 Some have a main 

 root growing straight 

 down into the soil, with 

 lateral roots developed 

 in vertical rows. This 

 is the tap-root system. 

 The number of these 

 rows varies. The 

 typical number in 

 dicotyledons is four ; 

 the beetroot shows two 

 sets of rows, and many 

 plants have five or 

 more. The older parts 

 of the root keep the 

 plant firmly fixed in 

 the soil ; the extremities 

 of the roots, where the 

 root -hairs are, absorb 

 FIG - '3;- R ; t ' System F 'V 4 -- P ; ot - syst T of the food-material. 



of bean. /, primary wheat, with grains of sand T 



root;/, lateral roots. attached to roots. ln Some C^SCS, par- 



ticularly in plants grow- 

 ing by the sea, the tap-root is very long, so as to penetrate the 

 deeper layers of the subsoil and thus get as much water as 

 possible. In prolonged drought the depth to which the roots 

 can penetrate may make all the difference to the possibility of 

 the plant surviving the long spell of dry weather. 



