LESSON 0.] MORPHOLOGY OF ROOTS. 33 



next winter, and sustain the third spring's growth, and so on ; 

 these plants being perennial (41), or lasting year after year, though 

 each particular root lives little more than one year. 



76. Many things which commonly pass for roots are not really 

 roots at all. Common potatoes are tuberous parts of stems, while 

 sweet potatoes are roots, like those of the Dahlia (Fig. 60). The dif- 

 ference between them will more plainly appear in the next Lesson. 



77. Secondary Roots, So far we have considered only the original 

 or primary root, that which proceeded from the lower end of the 

 first joint of stem in the plantlet springing from the seed, and its 

 subdivisions. We may now remark, that any other part of the stem 

 will produce roots just as well, whenever favorably situated for it ; 

 that is, when covered by the soil, which provides the darkness and 

 the moisture which is congenial to them. For these secondary roots, 

 as they may be called, partake of the ordinary disposition of the 

 organ : they avoid the light, and seek to bury themselves in the 

 ground. In Indian Corn we see roots early striking from the second 

 and the succeeding joints of stem under ground, more abundantly 

 than from the first joint (Fig. 42). And all stems that keep up a 

 connection with the soil such as those which creep along on or 

 beneath its surface are sure to strike root from almost every joint. 

 So will most branches when bent to the ground, and covered with 

 the soil : and even cuttings from the branches of most plants car. be 

 made to do so, if properly managed. Propagation by buds depends 

 upon this. That is, a piece of a plant which has stem and leaves, 

 either developed or in the bud, may be made to produce roots, and 

 so become an independent plant. 



78. In many plants the disposition to strike root is so strong, that 

 they even will spring from the stem above ground. In Indian Corn, 

 for example, it is well known that roots grow, not only from all those 

 joints round which the earth is heaped in hoeing, but also from those 

 several inches above the soil : and other plants produce them from 

 stems or branches high in the air. Such roots are called 



79. Aerial RoolS. All the most striking examples of these are met 

 with, as we might expect, in warmer and damper climates than ours, 

 and especially in deep forests which shut out much of the light ; this 

 being unfavorable to roots. The Mangrove of tropical shores, which 

 occurs on our own southern borders ; the Sugar Cane, from which 

 roots strike just as in Indian Corn, only from higher up the stem ; 

 the Pandanus, called Screw Pine (not from its resemblance to a 



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