146 GRAND PERIOD. [CH. VI 



SECTION B. Distribution. 



(171) Distribution of growth in roots 1 . 



Pick out a germinating bean with a root about 2 cm. 

 long. Make 5 marks, 2 mm. apart, on the root, the 

 first mark being 2 mm. from the tip of the root-cap. 

 To mark the root, the seed should be pinned to a cork 

 plate and the millimeter scale raised on a layer of cork 

 pinned to the first named cork plate like a step, so that 

 the graduated edge of the scale can be brought close 

 to the surface of the root to be marked. Eoots easily 

 suffer from dry air, it is therefore advisable to place each 

 seedling in water for a few minutes after it has been 

 marked, when it may be pinned, with 2 or 3 others 

 similarly treated, in a jar half filled with water, the roots 

 being in damp air. 



After 12 15 hours measure the distance between the 

 marks. The result will show that the most rapidly grow- 

 ing part of the root is a short distance behind the tip. 



(172) Distribution of growth in air-roots. 



In aerial roots the region of growth is of much greater 

 extent. Mark the air-root of an Aroid (e.g. Philodendron) 

 at intervals of 5 mm. for a space of 30 mm. from the tip ; 

 white paint, such as Aspinall's Enamel, is useful for 

 marking the dingy coloured roots of these plants. Measure 

 again after 2 days. 



1 See Sachs' Arbeiten, i. p. 414. 



