. A textbook of botany for colleges and universities ... Botany. ROOTS AND RHIZOIDS 5" resembling water roots, but, as soon as they enter the soil, they branch freely and produce hairs. Absorptive air roots. — Slriic- " 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 fea
. A textbook of botany for colleges and universities ... Botany. ROOTS AND RHIZOIDS 5" resembling water roots, but, as soon as they enter the soil, they branch freely and produce hairs. Absorptive air roots. — Slriic- " 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), tlie endodermis or inner- most layer of the cortex (w), 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 hair
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
