98 



HUTCHINSON'S POPULAR BOTANY 



The other elemental food substances are also found in the soil, and are 

 either dissolved by the water or by an acid sap excreted by the root-hairs. 

 This sap is a very necessary provision, as some of the substances essential 

 to vegetable life and growth are insoluble in water, and but for its timely 

 services the greater number of plants would be literally starved, and in 

 a short time disappear from the face of the earth. The powerful action 

 of this acid excretion may be shown by means of a simple experiment. 

 Let the perfectly smooth surfaces of two slabs of marble be spread with 

 sand to the depth of a quarter of an inch, and in one of the sand layers 

 sow some seeds of mustard and cress. Place both the slabs in a fairly 

 warm place and a good light, and water them occasionally till the plant-s- 

 on the seed-sown bed have 

 grown for a short time. On 

 cleaning off the sand from 

 the slabs it will be found 

 that the one which had the 

 sand only will be as smooth 

 as ever, while the other will 

 be covered with minute 

 grooves a kind of rough 

 etching of the root system. 

 In other words, the root- 

 hairs will have eaten their 

 way in the marble, channel- 

 ling out passages for them- 

 selves by means of the acid 

 sap. This experiment will 

 show how it is that large 

 trees are able to sink their 

 roots deep into the solid 

 rock, which may be literally 

 split to pieces by the subsequent growth of the tree's embedded roots. 



The nutrient substances are never taken up indiscriminately by the 

 plant. Not least among the many marvels of plant life is the mysterious 

 power vested in the root of selecting from the surrounding fluid the sub- 

 stances which it requires and rejecting others. Thus if you plant a pea 

 and a wheat-grain together in the same soil, the former will take care to 

 make the most of whatever of lime and its compounds the water of the 

 soil contains: while the latter, rejecting these, will absorb for itself the 

 silex or flinty matter. How this comes to pass we do not know, and 

 the wisest of savants can assign no reason ; but a power of selection 

 undoubtedly exists. 



From the fact itself we may learn a good deal. It is evident, for 

 instance, that the soil which is planted year after year with the same 



FIG. 132. VENUS' FLY-TRAP (Dioncea muscipula). 



The action of the leaves as an insect-trap was known as far back as the 



days of Linnaeus, but regarded only as an extreme example of vegetable 



irritability. It was not until Darwin had explained the Sundew that 



attention was drawn to the real purpose of Dionaea's movements. 



