86 THE PHYSIOLOGY OF PLANTS 



danger, should the watering be injudicious or the choice of 

 mould faulty (too heavy), of continuing and increasing the 

 decay. From the infected centre liquid products of decomposi- 

 tion are sucked up into the healthy tissues, cause the decay 

 of the stem, and in the end bring about the death of the plant. 



It is essential to use very loose moulds and small pots for 

 plants with sickly roots. 



First of all, no attention need be paid to a possible dearth 

 of nutritive substances which might occur after the plant has 

 made new roots. Such a want can easily be met by a subse- 

 quent transplanting. At present, the chief aim is to produce 

 as soon as possible a new supply of roots, and this necessitates 

 the preparation of a moderately moist soil, which can be kept 

 permanently well aerated. This even moisture must not be 

 brought about by repeated watering, but by a preservation of 

 the water contained in the soil in the first instance, and which 

 must be prevented from evaporating. This is most frequently 

 done by immersing the pots in a bed of cinders or sand, cocoa- 

 nut fibre, sawdust, tan, or other porous substances, which do 

 not easily decompose. It will be found of great advantage if 

 such a bed can be heated. Amateurs growing plants in an 

 ordinary room will find it advantageous to sink the pot in a 

 larger one containing river-sand. 



River-sand is in all cases preferable to sand derived from 

 pits, as the particles of the latter are always more liable to 

 adhere together, owing to the presence of clay, loam, and iron. 

 If river-sand is not obtainable, the pit-sand should always be 

 washed before use. High temperatures, which would be very 

 beneficial in frames or green-houses, should be avoided in the 

 dry atmosphere of a room. Sickly plants should receive more 

 broken potsherds below the soil than healthy plants, as they 

 require a better and more rapidly drained soil. 



The re-potting of a plant with a healthy ball of roots is 

 necessary when the soil becomes exhausted. A growing plant 

 often requires a larger amount of soil, the old soil being 

 entirely used up by the network of rootlets. The latter are 

 so closely pressed against the inside of the pot, that some are 

 always damaged in removing the pot. Now, it has become 

 the general practice to greatly increase this injury by ripping 



