148 



BOTANY 



PART I 



originate as in the stem. In some plants wide parenchymatous rays 

 are formed by the cambium opposite the strands of primary xylem 

 (Fig. 171 B). A cross-section of a root in which the secondary growth 

 has continued for some years can scarcely be distinguished from a 

 cross-section of a stem ; by careful examination, however, the character- 

 istic strands of primary xylem can be recognised in the centre of the 

 root. 



Repeated Formation of Cambium in Stems and Roots. Deviations from the 

 usual type of secondary growth as found in most Gymnosperms and Dicotyledons 



are met with in some cases. 

 These anomalous types are 

 characterised by differences in 

 the distribution and in the 

 activity of the cambium. 



In some Cycadeae and cer- 

 tain species of Gnetum among 

 the Gymnosperms and in the 

 Chenopodiaceae, Amaranta- 

 ceae, Nyctaginaceae, Phyto- 

 laccaceae, and some other 

 families of Dicotyledons, the 

 first ring of cambium, which 

 arose in the usual way, ceases 

 to function after a time. A 

 new zone of cambium forms 

 usually in the pericycle, ie. 

 external to the bast, or else in 

 tissue derived from the earlier 

 cambium. The new cambium 

 forms bast externally and wood 

 internally, these tissues being 



traversed by medullary rays. Its activity in turn comes to an end and its place 

 is taken by a new cambium formed outside this zone of bast. The process can be 

 repeated and leads to the production of concentric zones each composed of wood 

 and bast. This is seen, for example, in the transverse section of the stem of Mueuna 

 altissima, a liane belonging to the Papilionaceae which is represented in Fig. 172. 

 Such concentric zones of wood and bast are met with in some succulent roots 

 which persist for two or more vegetative periods. This is the case in the Beet 

 (Seta vulgaris), where the zones can be readily recognised with the naked eye on 

 cross-sections. They arise as described above, but, as in the case of the typical 

 secondary growth of other succulent roots, parenchymatous tissue which serves for 

 storage of reserve materials forms a large proportion of the newly-developed tissues. 



3. The Wood. A. Kinds of Tissue and their Functions. The con- 

 struction of the wood is complex, and in Dicotyledons it is usually com- 

 posed of three distinct types of tissue the walls of which are more or 

 less lignified. These are: (1) longitudinally-running strands of dead 

 VESSELS (Fig. 173 g, ty) ; (2) longitudinally-running strands of scleren- 

 chymatous fibres, WOOD-FIBRES (h), that are usually dead ; (3) STORAGE 



FIG. 172. Transverse section of the stem of Mueuna altis- 

 sima. 1, 2, 3, Successively -formed zones of wood; 

 1*. 2*, 3*, successively-formed zones of bast ; 3, 3* are 

 commencing to form within the pericycle. (f nat. size. 

 After SCHENCK.) 



