76 



between two groups of phloem and in this case the bundle 

 is bi-collateral. In cases where either the xylem or the 

 phloem is entirely surrounded by the other, the bundles 

 are called concentric. In some cases all the cells of a vascular 

 bundle develop into either xylem or phloem elements, but 

 in others certain cells remain meristematic, i.e. retain their 

 power of growth and division. In the former case the bundle 

 is said to be closed and in the latter open, the meristematic tissue 

 being called the cambium. On the outside of the vascular bundle 

 sclerenchymatous fibres are usually found, which often form a 

 more or less complete sheath to the bundle, 



Devei m ^' ^ vascu l ar bundle does not of 



of Vascuia? n course suddenly arise in the plant tissues with all its parts corn- 

 Bundles, plete, but is developed gradually, like all other parts of the 

 plant, from cells which at first are homogeneous. In the very 

 young plant embryo in the seed the vascular bundles are not 

 yet recognisable. As the embryo develops, however, certain rows 

 of cells grow longer than their neighbours and, dividing tangen- 

 tially, give rise to strands of narrow elongated cells, which are 

 destined to become the vascular bundles. The form of these 

 cells gradually changes as they complete their growth, some 

 elongate to form fibres and tracheids, the width of others 

 increases to form vessels, their walls become variously thickened 

 and pitted and their chemical composition is altered, many 

 cells lose their protoplasmic contents, while in other cases the 

 cross walls are more or less absorbed to form vessels and sieve- 

 tubes. This differentiation of a collateral vascular bundle 

 proceeds from the exterior of the bundle on both sides inwards 

 and the first formed elements of the xylem and phloem, which 

 are thus in this case the outermost, are called protoxylem and 

 protophloem, respectively. In the protoxylem annular or spiral 

 vessels or tracheids are found which do not occur in fche rest 

 of the xylem. 



The cells at the growing apices of the shoots and roots of 

 older plants are also practically homogeneous and meriste- 

 matic, just as they are in the plant embryo, and it is only 

 at some distance behind the so-called growing points that we 

 find the vascular bundles fully developed. 



