HISTOLOGY OF HIGHER PLANTS 65 



In certain places the green mesophyll cells are interrupted by 

 groups of cells containing no chlorophyll. At intervals beneath 

 the upper epidermis we see masses of large, thin-walled, colourless 

 cells forming a parenchyma or ground- tissue (par.), while beneath 

 the vascular bundles we find bands of very thick-walled cells (sk.) 

 which play an important part in the mechanical support and 

 strengthening of the leaf. 



The vascular bundles are surrounded each by a sheath of thin- 

 walled cells (sh.) containing numerous small starch grains. 

 Within this lie the bast or phloem and the wood or xylem, com- 

 posed of the elongated tubular elements through which the sap 

 circulates. The raw sap consists of water with mineral salts 

 in solution, and ascends through the xylem, while elaborated 

 sap, containing the proteids which have been manufactured in 

 the leaf under the influence of sunlight, descends through the 

 phloem. For our present purposes we may confine our attention 

 to certain of the xylem elements, known as the spiral and 

 annular vessels. These consist really of dead cell-walls, forming 

 long narrow tubes each composed originally of a row of cylindrical 

 cells placed end to end. In the course of their development the 

 transverse dividing walls between these cells are absorbed, the 

 protoplasm disappears, and nothing remains but a long hollow 

 tube whose walls are strengthened by spiral or annular thicken- 

 ings. Portions of these vessels are represented separately in 

 Fig. 26, D ; in A they are seen only in transverse section (ves.). 



Such are the principal kinds of tissue met with in a typical 

 flowering plant, and such is the way in which it carries out the 

 principles of co-operation, differentiation and division of labour 

 amongst its constituent cells. 



To most people it will probably appear that the fundamental 

 truth and general applicability of the cell theory are sufficiently 

 firmly established by considerations such as those with which we 

 have been dealing. It has, however, certain undoubted limita- 

 tions, and upon these limitations some biologists are inclined to 

 lay a good deal of stress. ^ 



Thus, from the point of view of the cell theory, we regard the 

 cell as the organic unit ; there are, however, units of a lower 

 order, of which the cell itself is composed. The chloroplastids of 

 one of the higher plants, for example, exhibit a good deal of 

 individuality, being capable of independent growth and multi- 

 plication, while, as we shall see later on, it is necessary for 



B. F 



