198 A TEXTBOOK OF GENERAL BOTANY 
extends around the outer end of the rays of the xylem, so that 
it forms a convoluted cylinder (Fig. 192). By the activity of 
the cambium, secondary phloém is formed on the inner side of 
the primary phloém. The secondary xylem is not formed next 
to the primary xylem (Fig. 193) but opposite the secondary 
phloém, while a pith ray 
is formed exterior to each 
of the primary rays of 
xylem (Figs. 193, 194). 
Although at first the 
cambium has the form 
of a convoluted cylinder, 
the convolutions are soon 
straightened out by un- 
equal rates of growth in 
different places, and the 
cambium, as seen in the 
cross section, becomes a 
regular circle (Fig. 194). 
The subsequent activity of 
the cambium of the root is 
similar to that of the stem. 




















































































































































































he A a it aan ae ell “ y) | } ; 
Fig. 195. Taeniophyllum, an epiphytic, leaf- SPECIALIZED ROOTS 
less orchid with roots which serve for ; 
roots are the absorption 
of water and mineral matter and the anchorage of the plant. 
Some parasites have specialized roots that produce haustoria 
(Fig. 8) which perform these functions. Moreover, roots, like 
other organs, may be specialized for functions which are not, 
in general, characteristic of the organ concerned. Such special- 
ized roots may be classified according to their functions under 
the following headings: photosynthesis (leaf function), sup- 
port (stem function), reproduction (seed function), storage, and 
aération. 
