GENERALIZED FORMS. 137 



174. Trichome.* The trichome is a surface appendage 

 consisting of one or more cells usually arranged in a row or 

 a column, sometimes in a mass. Its most common forms are 

 met with in (1) the Hairs of many plants. (See page 95. ) 



The other trichome forms are : 



(2.) Bristles, each consisting of a single pointed cell or 

 a row of cells, whose walls are much thickened and hardened. 



(3.) Prickles, like the last, bat stouter, and usually com- 

 posed of a mass of cells below. 



(4.) Scales, in which the terminal cell gives rise by fission 

 to a flat scale, which soon becomes dry. 



(5.) Glands, which are generally short, bearing one or 

 more secreting cells. 



(6.) Root-hairs, which are long, thin, single-celled (in 

 mosses a row of cells), and subterranean. 



(7.) Sporangia of Pteridophytes, some of whose interior 

 cells develop into reproductive cells (spores). 



(8.) Ovules of Phanerogams, one or more of whose cells 

 develop into reproductive cells (embryo sacs).f 



175. Boot. The root is that portion of the plant-body 

 which is clothed at its growing point with a root-cap. In 

 ascending through the vegetable kingdom roots are the 

 latest of the generalized forms to make their appearance, 

 and in the embryo they appear to be formed later than 

 caulome and phyllome. They present fewer variations than 

 any of the other generalized forms. The ordinary (1) Sub- 

 terranean roots of plants are typical. They differ but little 

 from one another in all the groups of the Pteridophytes and 

 Phanerogams. 



The other root forms are : 



(2.) Aerial roots, which project into the air, and often have 

 their epidermis peculiarly thickened, as in the epiphytic 

 orchids. 



(3.) Roots of Parasites, which are usually quite short, and 



* From the Greek #p<f , rptxoS, a hair. 



f It is held by some botanists that in some plants the ovule is " the 

 terminal portion of the axis," and that in others it is a leaf or part of a 

 leaf. 



