ROOTS AND RHIZOIDS 



5 11 



resembling water roots, but, as soon as they enter the soil, they branch freely and 

 produce hairs. 



Absorptive air roots. Struc- 

 tural features. Many plants 

 (known as epiphytes} grow on the 

 branches of trees, where the con- 

 ditions for absorption are much 

 poorer than in the soil. In many 

 tropical orchids and aroids (Ara- 

 ceae), the aerial roots possess 

 specialized absorptive organs 

 (fig. 729). These roots often are 

 silvery white except for the green- 

 ish tips, and their most distinc- 

 tive feature is the outermost or 

 epidermal layer, known as the 



FIG. 730. A cross section of an aerial 

 absorptive root of a tropical epiphytic orchid, 

 showing the velamen (v), the exodermis or 

 outermost cortical layer (#), the main body 

 of the cortex (c), the endodermis or inner- 

 most layer of the cortex (n), and the con- 

 ductive region (6). The velamen (v) repre- 

 sents the epidermis, and is composed of dead 

 cells, which, when dry, absorb water with 

 great rapidity; considerably magnified. 



velamen, which usually is a num- 

 ber of cells thick (fig. 730). At maturity the cells are dead and the walls 

 are variously thickened by reticulated or spirally arranged fibers (fig. 

 731). The outermost cortical layer, the exodermis, occurs beneath 

 the velamen as a sheath of cells with walls strongly thickened by cutin 



or cork. Some cells in this layer, 

 known as transfusion cells, remain 

 with relatively unthickened walls 

 and are said to serve as passage- 

 ways for water from the velamen 

 to the cortex (fig. 732). The cor- 

 tical cells resemble those in soil 

 roots, except that they contain an 

 abundance of chlorophyll, which 

 accounts for the greenness of the 

 roots when wet (fig. 733). 



Role. Water can be taken 

 up with rapidity by the velamen 

 when dry, the process being a 

 capillary phenomenon and com- 

 parable to the absorption of water 

 by blotting paper, and quite un- 

 like absorption by root hairs. 



731 "" 732 



FIGS. 731, 732, 733. Cells from various 

 regions of an aerial absorptive root of a 

 tropical epiphytic orchid, as seen in cross 

 section: 731, a velamen cell, showing the 

 characteristic fibrous thickening of the wall; 

 732, three exodermis cells, showing con- 

 siderable wall thickening, especially toward 

 the cortex (lower side in figure); 733, a cor- 

 tical cell, showing chloroplasts with included 

 starch grains ; all figures highly magnified. 



