824 A TEXTBOOK OF THEORETICAL BOTANY 



(i) Stilt Roots 



Adventitious roots sometimes form supporting stilts. A good example 

 is shown by the Maize, which develops a cluster of roots from the first one 

 or two nodes above ground level (Fig. 822). They grow obliquely down- 

 wards into the soil and give added support to the stem, which, with its large 

 leaves and heavy cobs, is very vulnerable to the efl^ects of wind. Stilt 

 roots are also well shown in Pandamis, (Figs. 823 and 824). Less 

 familiar, but much more striking, are the stilt roots of the Mangroves. 

 Several species of Rhizophora, Sonneratia and Avicennia make up the tidal 

 woodlands in tropical bays and estuaries. All these trees are supported by 

 scaffoldings of stilt roots, which may cover many square yards round each 

 tree, making a formidable entanglement over the ground (Fig. 825). Not 

 infrequently the base of the stem rots away, so that the tree is left perched 

 on its stilts above the mud, like a giant vegetable crab. 



(7) Prop Roots 



Prop roots are those formed from the main branches of a tree. They 

 grow straight downwards to the soil, which they enter and form underground 

 branch roots. As they become thickened they give support to the branches, 

 which are thus able to continue their horizontal growth, producing more 

 prop roots at intervals. The best example of this is Ficus benghalensis, the 

 Banyan Tree (Fig. 826). Banyan is the vernacular name for a merchant, 

 and it was given to this tree because it was the customary shelter for itinerant 

 pedlars, who set up stalls in their shade. Some of the biggest specimens 

 have been used as shelters for whole villages of native huts. 



{k) Thorn Roots 



It only remains to mention two minor but curious modifications. The 

 first is the formation of thorn roots at the base of the stem in several species 

 of Palms. As in the case of stem thorns, these begin as normal structures 

 but their tissues soon become densely sclerotic, including their apices, which 

 then cease growth. In addition to root thorns, which are modifications of 

 the roots themselves, some plants bear root spines which are outgrowths 

 from the roots, e.g., the Palm, Iriartea exorhiza, the big spiny stilt roots of 

 which are used as graters in Panama. 



(/) Cortical Roots 



The second is the case of internal or cortical roots in several Mono- 

 cotyledons, including especially Tillandsia (Fig. 827) and Vellozia. These 

 roots grow directly downwards from their point of origin in the pericycle 

 of the stem, penetrating the cortex as they go. Eventually they emerge from 

 near the base. In Vellozia, which is common on exposed rocky places in 

 south Brazil, these roots are not indeed truly internal. It is more correct 

 to say that they actually make up a pseudo-cortex, covered and protected by 

 a sheath of persistent leaf bases. The intensely dry habitat would no doubt 

 prevent the growth of exposed aerial roots. A comparison suggests itself 



