56 PRACTICAL BOTANY 



several different thin-walled tissue-elements, which are, 

 however, difficult to distinguish in transverse sections : they 

 are 



a. Sieve-tubes, which are nearly circular in section, and 

 usually of larger cavity than the other constituents. 



b. Bast-parenchyma : cells often arranged in more or less 

 regular radial rows : certain of the cells differ from the rest in 

 containing one or more crystals. 



The nature of these several tissues will be more successfully 

 studied in longitudinal sections. 



6. The cambium consists of thin-walled cells arranged, as 

 in the Sunflower, in radial rows, which may often be traced 

 outwards into the phloem, and inwards into the xylem : the 

 cells have copious protoplasm, in which an elongated nucleus 

 may often be observed. 



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. 



b. Xylem fibres or wood-prosenchyma, consisting of ele- 

 ments with much smaller cavity, little or no protoplasm, 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 connecting two consecutive medullary 

 rays laterally. 



The cells of those parts of the medullary rays which are in 

 the xylem are thick-walled, lignified, and pitted : they have 

 protoplasmic contents and starch : they are elongated in a 

 radial direction. Note that they have special cambium-cells, 

 differing in form from the ordinary cambium (compare Fig. 7, 

 A). In the phloem the cells are thin-walled (cellulose), and 

 have plentiful protoplasm. 



8. The pith. In the peripheral part the cells have thick, 

 lignified, pitted walls, and a protoplasmic body with starch (at 



