ROOT, STEM, AND LEAF 15 



In the bean, pea, and corn, on the other hand, since the cotyle- 

 dons cannot serve as foliage leaves, the later leaves must be 

 pushed forward rapidly. In the bean the first pair are already 

 well formed in the seed. In the pea they cannot be clearly 

 made out, since the young plant forms several scales on its stem 

 before it produces any full-sized leaves, and the embryo contains 

 only hypocotyl, cotyledons, and a sort of knobbed plumule, well 

 developed in point of size, representing the lower, scaly part of 

 the stem. 



20. Root, stem, and leaf. By the time the seedling is well 

 out of the ground the plant body, in most cases, possesses the 

 three kinds of vegetative organs, or parts essential to growth, of 

 ordinary seed plants ; that is, the root, stem, and leaf, or, as they 

 are sometimes classified, root and shoot. All of these organs 

 may multiply and increase in size as the plant grows older, and 

 their mature structure will be studied in later chapters ; but 

 some facts concerning them can best be learned by watching 

 their growth from the outset. 



21. Elongation of the root. "We know that the roots of seed- 

 lings grow pretty rapidly from the fact that each day finds them 

 reaching visibly farther down into the water or other medium 

 in which they are planted. A sprouted Windsor bean in a ver- 

 tical thistle tube will send its root downward fast enough so 

 that ten minutes' watching through the microscope will suffice 

 to show growth. 



22. Root hairs. Very young seedlings of the grains, or of 

 mustard or red clover, afford convenient material for studying 

 root hairs. These are most abundantly developed when the 

 seed is sprouted in air that is not very moist. Only a certain 

 zone of the young root is covered with live hairs ; the younger 

 portions have not developed them and the older portions show 

 only dead ones. Examination with a good lens or a low power 

 of the microscope shows the gradual lengthening of the hairs, 

 from very young ones near the root tip to full-grown ones 

 farther up. 



