30.] 



CHAPTER II. THE TISSUES. 



147 



celled meristem. It is, however, frequently the case, especially when the stem 

 or root is rather bulky, that the distinction of the initial layers is not clear, and 

 that they seem, in fact, to have to a greater or less extent a common origin, 

 and various such " types " of structure of the growing-point have been 

 described ; these " types " are, however, only variations of the relations as 

 described above, due to irregularities of growth. 



Fi&. 111. Median longitudinal section through the growing-point of the root of Hordeum 

 vulgare (Barley): r roct-cap ; fc initial cells of the dermatogen of the many-layered epider- 

 mis ; d-en cortex ; d epiblema with mucilaginous external layer of cell- wall c ; i cortical 

 tissue with intercellular spaces ; en endcdermis ; the whole periblem (pr) is derived from 

 the single layer of two initial cells at the apex ; pi plerome ; a row of cells which give rise 

 to a large central wood-vessel. (After Strasburger : x 180.) 



I). Growing -points with a single apical cell are to be found in 



