32 THE TISSUES OP PLANTS 



appearance in their external configuration, constitute the 

 substance of each of these organs. At this stage of growth 

 the cells are all apparently the same, and endowed with 

 similar functions; but that this is not really the case, is 

 shown by their subsequent behavior in their after develop- 

 ment. It is only in plants which are very low in organi- 

 zation, such as algae and lichens, that the cells permanently 

 retain this primitive and uniform appearance; in vegetation 

 of a higher grade, this uniformity speedily disappears, and 

 the individuality of the cells becomes manifest as growth 

 progresses. Whilst some of them continue spherical, 

 others take a much higher degree of development, and be- 

 come gradually transformed into woody fibre, vascular 

 tissue, and spiral vessels. 



We say that certain determinate cells only, thus change 

 their character. This is apparent on the cross section of 

 the stem of any herbaceous exogen, which has just begun 

 to grow, and to unfold its first sets of leaves. It will be 

 seen that the cells in the centre and towards the circum- 

 ference of the stem, which form collectively the pith and 

 the bark, together with those of the medullary rays, are 

 only slightly altered by mutual pressure from the spherical 

 form. Those, on the contrary, which constitute the wood, 

 and which occupy an intermediate position between the 

 bark and the pith, are so changed in appearance that it 

 seems at first impossible to refer them to the same common 

 type. If we examine a longitudinal section of the stem, 

 the nature and extent of the transformation which the wood 

 cells have undergone, will be rendered more apparent. It 

 will be seen that the fibrous portion of the wood consists of 

 elongated, and extremely attenuated cells, which taper to 

 either extremity and lie together in bundles, and that 

 there are intermingled with these fibres several varieties of 



