96 PRACTICAL BOTANY 



Note that the tangential walls are thinner than the radial 

 walls ; also that the radial diameter of the cells is less than the 

 tangential. These facts, together with the arrangement of the 

 cells in radial rows, point to a sequence of divisions, by walls 

 parallel to one another, in a tangential direction. If careful com- 

 parisons of a number of different radial series be made, it will be 

 found that the arrangement is such as would result from the 

 action of Sanio's law of cambial division (compare Fig, 9, A, 

 on p. 107). 



The structure of the cambium may be very well demonstrated 

 in young stems of Clematis. 



7. The xylem also consists of several different tissue- 

 forms, all of which have lignified walls (note their 

 reactions) : they are 



a. Vessels, easily recognized by their large cavity, 

 and by the absence of any protoplasmic body. They 

 occur, singly or in groups, scattered through the xylem. 



It may be found that the cavity of some of the vessels is filled 

 with a cellular tissue. This is especially frequent in the part of 

 the xylem-ring nearer to the centre : the name thylose is given 

 to such cells (see above, p. 75). 



1}. Xylem-fibres or wood-prosenchyma, consisting 

 of elements with much smaller cavity, little or no proto- 

 plasm, and thick walls. 



c. Xylem-parenchyma, recognized by the presence 

 of a protoplasmic body, and (at all events in winter) of 

 starch-grains. The cells of this tissue are usually 

 grouped round the vessels, and often form bands con- 

 necting two consecutive medullary rays laterally. 



The cells of those parts of the medullary rays 

 which are in the xylem , are thick- walled, lignined, 

 and pitted : they have protoplasmic contents and 



