1 86 Practical Plant Biology. 



bundles. The first part of the bundle to be differentiated is 

 the innermost part of the wood. To form it the inner cells of 

 the procambial tract elongate and become tubular, spiral and 

 annular bands are laid down inside their walls while the proto- 

 plasmic contents disappear and the walls become lignified. The 

 spiral and annular tubes developed first from the procambium 

 form a continuous thread of tissue called the protoxylem, running 

 on the inner side of the wood strand. Nearly as soon as the 

 formation of the protoxylem on the inside of the procambial tract, 

 the differentiation of a tissue called the protophloem takes place 

 on its outside. The change here of the procambial cells is not 

 so marked, for the walls remain cellulose and develop no special 

 thickenings. A thin film of cytoplasm is still retained, but the 

 nucleus breaks up and disappears. Between the protophloem 

 and protoxylem there is left over, a thin layer of undifferentiated 

 cells. These form the cambium. The inner cells of the cambium 

 develop the characteristics of woody capillary tubes, and the 

 outer cells those of sieve-tubes, while the cells forming the middle 

 of the layer divide longitudinally and give rise to new un- 

 differentiated cambium cells. In this way the thickness of the 

 strand of wood is increased by additions on its outside, while 

 the bast receives additions on its inner surface. 



The growth in thickness of the two strands of the conducting 

 tract naturally stretches the medullary rays in a radial direction. 

 Apparently in response to this stimulus the cells of the rays, equi- 

 distant from the pith with the cambium in the conducting tracts, 

 begin to divide, and to add to the radial length of the medullary 

 rays. This dividing layer in the rays is called the interfascicular 

 cambium, in contradistinction to the cambium of the vascular 

 bundles, which is called fascicular cambium. The wood and the 

 bast which are formed before the development of the interfascicular 

 cambium are called primary wood and primary bast. Wood and 

 bast formed subsequently to that stage are secondary. The fascicular 

 and interfascicular cambium now form a continuous sheet outside 

 the wood intersecting the medullary rays. This sheet lays down 

 on its inner surface wood-tubes and medullary-ray-cells, and on its 

 outer surface sieve-tubes and medullary-ray-cells. These new 

 elements sometimes correspond to those already laid down in the 

 primary arrangement of tissues, but sometimes they do not. In 

 the latter case medullary-ray-cells laid down radially opposite the 

 primary wood strands initiate a ray which does not extend as far 

 as the pith and does not reach the cortex. Such rays are called 

 secondary, in contradistinction to those which extend from the 



