92 STRUCTURAL BOTANY 



from the peculiarity in the endodermis, the structure 

 quite agrees with that of a simple root in the higher 

 plants. 



d. Growing-Points and Branching 



The growing-points of Equisetum afford perhaps the 

 very best examples of growth by means of a single apical 

 cell, by the divisions of which all the tissues arise. The 

 apex of the stem is acutely conical (see Fig. 42), and the 

 top of the cone is occupied by the large apical cell, which 

 has the form, so common in apical cells, of an inverted 

 three-sided pyramid, of which the curved base is free, 

 while the three sides are adjacent to the surrounding meris- 

 tematic tissue. Divisions take place in the apical cell 

 by walls formed in succession parallel to each of its three 

 sides ; each segment cut off is then divided into two by 

 a wall parallel to the first. 



The cells thus formed are again divided by approxi- 

 mately radial walls, and then for the first time division 

 takes place in a plane parallel to the external surface of 

 the growing-point. We now have an outer and an inner 

 set of cells. The former, by their further growth and 

 subdivision, give rise to the whole of the vascular tissue, 

 cortex and epidermis, the inner cells only form the pith, 

 which in the main stem soon becomes hollow. There is 

 here no trace of the distinct layers giving rise to epi- 

 dermis, cortex, and stele, such as are sometimes to be 

 recognised in Flowering Plants. The ring of vascular 

 bundles is only marked out at a long distance below the 

 growing-point. About the fifth internode from the apex 

 we find a small-celled zone of tissue, derived from the 

 inner part of the outer layer. This zone gives rise to 

 the vascular bundles, and to the medullary rays between 



