B60 MANUAL OF BOTANY 



of origin, but also by not extending to the pith on the one hand 

 or to the peric3'cl6 on the other. Their extent in a vertical direc- 

 tion also varies considerably in different cases. 



The medullary rays, both primary and secondary, usually 

 v^iden out in the bast in consequence of the stretching of the 

 tissue brought about by the continuous increase of the central 

 mass of wood. This leads to radial divisions of the cells of the 

 nreduUary rays towards their outer ends. 



The cells formed by the cambium undergo similar trans- 

 formations to those of the procambium. The tissue of the two 

 meristems is somewhat different, however, the procambium cells 

 being elongated and narrow with square ends, while those of the 

 cambium are prismatic. The behaviour of the two is different, 

 the divisions of the procambial cells taking place in three planes, 

 those of the cambium chiefly tangentially, but to a slight extent 

 radially as well. 



The activity of the cambium is greater towards the interior 

 of the stem than outwards, so that much more wood is formed 

 than bast. 



The cells when first cut off from the cambium are thin- 

 walled and hardly distinguishable from the merismatio cells. 

 Gradually they become changed to the permanent form. Those 

 which are to form xylem parenchjina divide transversely, 

 becoming cubical instead of prismatic. Those destined to be 

 tracheids or segments of vessels increase considerably in 

 diameter, displacing and compressmg the adjacent cells. They 

 thicken regularly or irregularly, as before described. The fibres 

 which originate from the cambium exhibit a curious mode of 

 growth, their upper and lower ends, which are, like those of 

 the cambial cells, somewhat pointed, gi'owing past those of the 

 cells above and below them, which behave similarly. Thus the 

 separate fibres extend themselves between their neighbours, 

 and often attain considerable length. Their transverse diameter 

 does not increase as in the case of the vessels. This mode of 

 behaviour is known as sliding groivth. It is seen not only in 

 the young stem, but often in cases where growth in thickness is 

 not accompanied by growth in length, when it causes curious 

 displacements of the tissues. 



In the Conifers the secondary wood formed by the cambium 

 consists of fibrous tracheids whose radial walls are furnished 

 with bordered pits {fig. 638). 



The cells of the secondary Vast include chiefly parenchyma and 

 sieve tubes. The latter show a little peculiarity in their develop- 



