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OUTLINES OF EVOLUTIONARY BIOLOGY 



plants, where the various meristematic or actively dividing 

 tissues remain in an undifferentiated embryonic condition and 

 give rise to those additions to the permanent tissues whereby 

 growth is effected. Such actively dividing meristem is found at 

 the growing points of stems and roots, where it serves to bring 

 about growth in length, and in the cambium, which serves, by 

 the addition of new elements to the wood and the bast, to bring 

 about growth in thickness. 



The microscopic appearance of such a meristematic tissue, 



B. 



FIG. 33. Part of a longitudinal Section of the actively growing Boot of a 

 Hyacinth (Qaltonia candicans} showing the Nuclei of the Cells in various 

 stages of mitotic Division, A X 280 ; B X 640. (From photographs.) 



when suitably stained and prepared for examination, is shown in 

 Fig. 33, taken from photographs of part of a longitudinal section 

 of the growing point of the root of a hyacinth (Galtonia candicans). 

 The cell-walls are as yet thin and inconspicuous and filled with 

 dense protoplasm, while the conspicuous nuclei exhibit all stages 

 of mitosis, the whole forming a striking contrast to the dead 

 tissues, such as cork and wood, of which the bulk of many plants 

 is made up, and which consists merely of cell-walls without any 

 protoplasmic contents (cf. Figs. 6 and 7). 



Mitosis in the cells of the higher plants is usually, though by 



