THE GROWING POINT 283 



site the point at which it first arose. Thus the stem comes to be enclosed by 

 successive leaf sheaths. 



The lateral branches arise practically in the same way as the leaves, so that we 

 need not discuss them further. The root, however, from its special peculiarities 

 claims our attention. The growing point of the root is always intercalary and 

 forms new and different structures in two directions. The apex of the root is 

 always covered by a rootcap which protects the delicate growing region. The 

 rootcap, so far as its function is concerned, may be compared with the other 

 protective adaptations to which we drew attention in speaking of the growing 

 point of the stem. It consists of simple parenchymatous tissue which is especi- 

 ally well developed at the extreme apex of the root, but also encloses the sides 

 of the root for some distance. Its cells are short lived, but they are constantly 

 renewed by the growing point itself. In spite of this renewal the rootcap does 

 not increase in size because the older cells die off in front and are cast off as 

 new ones are formed behind. On the other hand, the root itself exhibits con- 

 tinued increase in length at the apex owing to the activity of this same grovying 

 point. In detail there are important differences in structure among root apices, 

 but all that concerns us is the general principle on which they are formed, so 

 that we may confine ourselves to one example, e. g. the longitudinal section of 

 the root-apex of Hordeiim shown at Fig. 80. We see that the growing point 

 is constructed in a manner similar to that of the stem, especially as regards 

 the regularity of the periclinal divisions. The formative layers of the rootcap 

 are seen at k. (As regards the remaining features see the explanation beneath 

 the figure.) If, as in ferns, an apical cell occurs, the cutting off of segment cells 

 follows entirely the rule which governs segmentation of the apical cell of the 

 stem, save that segments are cut off on the fourth or outer side of the pyramid 

 also, and from these the cap is formed. 



The difference between root and stem comes out especially in the branching. 

 The root produces no leaves and no buds but lateral roots only, which are 

 quite like the main root. Hence branching in the root is extremely uniform. 

 The point of origin of the lateral root branches is, however, quite unique. 

 ' Root buds', comparable with stem buds, do not exist. The terminal part of 

 the root is quite free from lateral roots for a considerable distance, and when 

 they do appear, some way back from the growing point, they burst out from 

 within, through tissues already full grown ; in other words, roots arise endo- 

 genously, while leaves and buds arise exogenously. At first sight it might appear 

 as though the lateral roots could not be derived from the growing point of the 

 main root, as though in short we had here an exception to the rule which we 

 have established for the stem, viz. that every new growing point is a portion of 

 a previously existing one. When, however, we examine a transverse section of 

 a fully-developed root we notice a layer of cells lying within the parenchymatous 

 cortex separated off by the ' endodermis ', and which, under the name of ' peri- 

 cycle', surrounds the central vascular system. This pericycle may be traced 

 right up to the growing point, and its cells have the peculiarity of retaining for 

 long the characters of embryonic tissue. When the whole of the suiTounding 

 cells have been transformed into permanent tissue they remain still capable of 

 division. The pericycle is in short a residuum of the growing point and it is 

 from it that the lateral roots arise. Their origin from the pericycle is illus- 

 trated in Fig. 81. 



After the young root has been laid down it grows out to the exterior by 

 dissolving and mechanically destroying the cortical tissue before it. We shall 

 see in our next lecture wherein lies the biological significance of the late forma- 

 tion of lateral roots ; at present it is sufficient to note the fact that in spite of 

 their late formation we can still refer their origin to the growing point of the 

 chief root, and that the rule enunciated above, which we may, with Sachs, term 



