13.] CHAPTER II. SPECIAL MORPHOLOGY OF THE MEMBERS. 



63 



Adventitious roots are most abundantly produced when the normal root- 

 system is feeble. They are usually developed by the formation of growing- 

 points in members which are adult ; but in some cases they spring from the 

 growing-point of the shoot. The roots of Marattiaceae, of Lycopodium Phleg- 

 maria and other species, of Gunnera, of Nuphar, all take origin from the 

 growing-point of the shoot ; they may either at once grow out to the surface, or 

 they may grow down through the tissue of the stem, emerging at its base 

 (Lycopodium), or between some of the lower leaf-scars (Marattia). In Neottia 

 Nidus Avis the adventitious roots are largely developed from the intercalary 

 gffowing-point of the rhizome. 



The form of the root is usually cylindrical ; when it is very 

 delicate, as in Grasses, it said to be fibrous ; in some plants, as 

 mentioned above, it is hair-like. The primary or the secondary 

 roots may become much swollen, serving as depositories for 

 nutritive substances ; the Turnip, 

 the Carrot, the Beet, the Radish, 

 have swollen primary roots ; the 

 Dahlia has swollen secondary roots. 



Various terms are employed to designate 

 the different forms of swollen roots ; that of 

 the Turnip is termed napiform ; that of the 

 Carrot, conical ; that of the Kadish, fusi- 

 form or spindle shaped ; those of the Dahlia 

 and of some terrestrial Orchids, tuberous. 



Many plants have aerial roots 

 which are peculiar both morpho- 

 logically and physiologically. The 

 roots of epiphytes, that is, plants 

 (mostly Orchids, and Bromeliacece) 

 which grow on trees without, how- 

 ever, being parasitic, never reach 

 the ground, but serve as a means of 



attachment : they frequently contain chlorophyll and serve as 

 organs of assimilation, especially in Podostemaceae. Some plants 

 climb by means of aerial roots (e.g. Ivy, Tecoma radicans), which 

 are developed on the ventral surface of the dorsiventral stem, 

 and adhere closely to the tree-trunk or wall on which the plant 

 is climbing. 



In some rare cases the aerial root is a tendril, as in Vanilla 

 aromatica, Lycopodium rupestre and other species, and in some 

 Melastomacese (Medinilla radicans, Dissochaeta). 



FIG. 39. The lateral roots n aris- 

 ing entfogenously from the pericycle 

 of the tap-root of Vicia Faba (Longi- 

 tudinal sec. mag. 5 times). / Axial 

 cylinder (stele) ; r cortex of the main 

 root ; 7i root-cap of the lateral root. 



