92 REGENERATION 



the stem is suspended in moist air than when suspended under 

 water. In this case roots appear in each node, but some- 

 times without respect to gravity (Fig. 63). A similar disap- 

 pearance of the effect of gravity on the formation of organs was 

 noticed in Chapter V when leaves were put under water. The 

 explanation given there may hold also for stems, namely that 

 the effect of gravity is primarily due to tissue sap collecting on the 

 under side of a stem suspended horizontally. This causes an 

 acceleration of growth on the under side which in its turn causes 

 a flow of sap of the whole stem towards the under side, thereby 

 suppressing the growth of roots on the upper side of the stem. 

 When, however, the stem is under water the collection of the 

 water of the tissue sap on the lower side of the stem can no longer 

 cause a more rapid growth of roots on this side, since the tissues 

 on the upper side of the stem have also an abundance of water. 



