io8 THE LIFE OF THE PLANT 



In order to discover how the root fulfils its function, 

 we must begin by studying its structure. 



In external appearance a root belongs to one or other of 

 two clearly differentiated types. It either grows vertically 

 downward as a single continuous trunk which gradually 

 tapers to a fine thread, e.g. the beet, the carrot, flax ; 

 or else it branches at once, almost at the surface of the 

 soil, into a tuft of thread-like fibres, such as those of 

 our cereals — rye, wheat, and so on. Roots of the former 

 kind are called tap-roots ; of the latter, fibrous-roots. 

 These two extreme types have many modifications, 

 but they underlie all the variety of forms presented by 

 this organ. 



Every root, whether tap-root, single fibre, or lateral 

 root, grows in the same way as the stem, by elongating 

 and expanding at the apex. But we notice a great 

 difference when we compare the apex of the stem with 

 that of the root. If we remove all the leaves of a bud, 

 thus laying bare the apex of the stem, the so-called 

 ' growing point,' we find that it is the youngest and also 

 the most tender part of the stem, consisting of minute, 

 undeveloped cells. If we examine at the same time 

 the apex of the root, which is generally bare already 

 since there are no leaves to remove, we sometimes see 

 with the naked eye, but better with a lens, and better 

 still with a microscope, that it presents an untidy and 

 ragged appearance. It seems to be covered with a cap 

 consisting of several rows of cells, the outer of which 

 have already lost coherence, and only stick together by 

 means of a kind of mucilage. This cap is nothing but 

 the external dying layers of tissue, which cover and 

 protect the young and tender tissue of the growing 

 point lying under them (fig. 32). ^ Sometimes this cap 

 can be pulled off the root-tip like a glove. The physio- 

 logical purpose of this organ is easily understood ; it 



'■ Fig. 32 shows under a low power the growing point of the root with 

 its cap. 



