PLANT ORGANS AND ORGANISMS 21 7 



with an accompanying release of energy, which latter is necessary to 

 life and growth. 



The combined pericarp and spermoderm bursts opposite the tip of 

 the radicle, and the radicle, piercing through the cotyledonary sheath, 

 protrudes. The cleft in the coat lengthens to the point opposite the 

 tip of the plumule, which also protrudes after bursting through the 

 cotyledonary sheath. The radicle, next, grows downward into the 

 soil forming the primary root, and develops upon itself secondary or 

 lateral roots, all of which give rise to root-hairs just above their root 

 caps. Additional lateral roots emerge above the scutellar region 

 which ere long attain the size of the first or primary root. The cau- 

 licle, carrying upon its tip the plumule, elongates and forms the stem; 

 the leaves of the plumule spread out and turn green to function as 

 foliage leaves. The perforated cotyledonary sheath grows out sur- 

 rounding both the root and the stem for a portion of their length. By 

 this time all or nearly all of the nourishment stored in the endosperm 

 has been absorbed and assimilated by the young seedling and the 

 coat and scutellum, left behind, gradually decay and disappear. 

 The root-hairs absorb nourishment from the soil, the green leaves 

 build up carbohydrates, prop-roots make their appearance at the 

 first node (joint) above ground, and the seedling grows larger. 



Gross Structure of a Dicotyl Seed, Phaseolus lunatus (Lima Bean). 

 The Lima Bean Seed shows a flattened-ovate to somewhat reni- 

 form outline. Externally it exhibits a polished seed coat which is 

 perforated on its basal side by a minute pore called the micropyle or 

 foramen. Just below this pore will be noted the hilum or scar which 

 represents the point of detachment from thefuniculus or stalk, which 

 connected the seed during its growth with the wall of the fruit. 

 Upon soaking the seed in water, it is possible to remove the seed coal 

 or spermoderm. This done, the embryo will be exposed. The two 

 fleshy cotyledons are first seen. Upon spreading these out, convex 

 sides down, the rest of the embryo, consisting of a thin leafy structure 

 surrounding a bud and called the plumule, the caulicle or rudimen- 

 tary stem and in line with the latter, the radicle, or rudimentary 

 root, will be seen. 



Histology of the Lima Bean Seed. In transverse sections, the 

 following microscopic structures will be evident: 



