410 



BOTANY 



two-fifths, three-eighths, etc. (For an extended discussion of leaf- 

 forms and arrangement, see Gray, " Structural Botany.") 



Bracts, and the organs of the flower, are also foliar structures. 



The Root (De Bary, 2 ; Van Tieghem, 20) 



The primary root in the Dicotyledons, like that of the Gymno- 



sperms, is a continuation of the hypocotyl, and often persists as a 



tap-root. 



The apex of the root in most Dicotyledons shows three sets of 



initials (Fig. 388). Overlying the apex of the plerome cylinder is a 



single initial layer, or 

 two, which belongs to 

 the periblem. Outside 

 this is a common initial 

 layer for root-cap (ca- 

 lyptrogen) and derma- 

 togen. The structure of 

 the older roots is essen- 

 tially the same as in the 

 root of the seedling. 



The secondary roots 

 arise from the peri- 

 cycle, and very early 

 show the differentiation 

 of the three primary 

 tissue-meristems. 



Like the stem and 

 leaves, the roots of Di- 

 cotyledons show vari- 

 ous modifications, such 

 as root-tubers (Dahlia), 

 root-tendrils (Rhus toxi- 



codendrori), aerial roots (Ficus sp., Mangrove). (See Chapter II.) 



FIG. 389. Diagrams to show normal secondary thick- 

 ening in the roots of Dicotyledons ; cor, cortex ; 

 en, endodermis; p, pericycle; c, cambium; a; 1 , 

 primary, x 2 , secondary, wood ; ph 1 , ph 2 , primary and 

 secondary phloem. 



Trichomes 



Epidermal outgrowths, or Trichomes, are better developed, as a 

 rule, in Dicotyledons than in Monocotyledons. These trichomes 

 show great variety, ranging from delicate downy hairs to coarse 

 bristles or scurfy scales. Glandular hairs are very common in 

 many Dicotyledons, and are probably mainly defensive, either repel- 

 ling animals by their strong-scented secretions, or in some cases 

 e.g. Salpiglossis actually capturing Aphides and similar small in- 

 sects which would injure the plant. The dense felted masses of hairs 



