THE STRUCTURE OF PLANTS. 



3 



plants having embryos of this kind are termed Dicotyledonous (i. e. 

 having two cotyledons or seed-leaves). There are many plants in which 

 the embryo has but one cotyledon ; this is the case with Wheat, Indian 

 Corn, the Onion, Lily, &c. ; such plants are Monocotijledonous (i. e. 

 having one cotyledon). This is an important distinction, and divides 

 all our flowering plants into two great classes — Dicotyledonous and 

 Monocotyledonous plants — which are further distinguished by impor- 

 tant diflerences in their stem, leaves, and flowers. In the Fine Family, 

 the embryo has several cotyledons in a whorl — Polycotyledonous, 

 (Fig. 238). 



9. The Root or Descending Axis, is that portion of the radicle which 

 grows downwards, fixing the plant to the soil ; its office is to absorb 

 nourishment from the earth, and to this end it is provided with an 

 extended surface by being generally subdivided into branches and sup- 

 plied with multitudes of delicate Ahres or root-hairs. The root of a 

 plant, which springs from the seed, makes its growth and produces 

 flowers and seed all in one year, is called annual. When the plant re- 

 quires two years to complete its career, it is biennial ; and when it lives 

 through a number of years, it is perennial. 



10. The various forms which roots present, are produced either by the 

 branching of the principal root, or by the enlargement of it and that of its 

 branches. W^here the main root continues distinct and unbranching, or 

 sends ofl" only occasional branches, a tap-root is formed ; ordinarily, how- 

 ever, the main root is lost in its numerous branches, or many roots 

 start from the lower end of the radicle, and we have a cluster of roots. 

 Annual roots are very much divided into numerous thread-like branches ; 

 such are termed fibrous roots. In biennial plants, the first year is occu- 

 pied in storing up nourishment to be expended in producing flowers and 

 seed in the following year ; this is frequently deposited in the root, hence 

 the roots of biennials are usually thick and fleshy. If such roots taper 

 regularly downwards (as in the Parsnip and Carrot), they are conical. 

 if they taper both upwards and downwards (the Long Radish, for 

 example), we have a spindle-shaped or fusiform root. When much en- 

 larged laterally, so as to be broader than long, the root is turnip-si taped 

 or napiform. In these forms the branches are small and hair-like. In 

 some perennial roots, those where the stem dies down annually, there is 

 often an accumulation of nourishment and the roots become tuberous, as 

 in the Sweet Potato. 



11. Under favorable circumstances, roots may spring from any por- 

 tion of the stem and branches. When a branch lies along the earth, or 

 when a cutting is placed in the soil, roots are given out ; these are 

 termed secondary roots. Some stems throw out roots even at a great 

 distance from the earth (aerial roots), which serve in some cases only as 

 supports to the stems of climbing plants, adhering to rocks, the trunks 

 of trees and other objects, as in the Ivy and Poison Oak,— or they at 

 length reach the earth and help sustain the plant, as in the Indian Corn, 

 which often throws out roots from the lower part of the stem, at some 

 distance from the surface of the earth. In Paravtes (those plants which 



