D1V. II 



PHYSIOLOGY 



287 



turgescence of the individual cells. The tissue tension presents a certain 

 resemblance to the turgescence of the cell ; this is most evident in the 

 typical stem. Just as the cell sap distends the cell wall by its osmotic 

 pressure, the expanding pith stretches the cortical tissues. Increased 

 resistance to deformation and increased rigidity result from the 

 stretching of the cortex, just as they do in the cell from the stretching 

 of the wall. 



The tissue tension ceases as all the cells attain the permanent 

 mean length dictated by the size of the organ. Sometimes, however, 

 certain cells after attaining their greatest 

 length exhibit a considerable contraction 

 associated with an alteration in shape. 

 This occurs often in roots when the tissues 

 of the cortex nd of the central portion 

 are thrown into folds by the contraction 

 of the tissue that lies between them. The 

 significance of this contraction of roots, 

 which may lead to a shortening of the 

 fully-grown structure by 10-70 per cent, 

 is very great. Thus it is due to it that 

 the leaves of many " rosette plants," in 

 spite of the continued growth in length 

 of the stem, remain always appressed to 

 the soil. It determines and regulates the 

 penetration of many tubers and bulbs to 

 a definite depth in the soil. It increases 

 the fixation of the plant in the soil, since 

 greater stability results from tense than 

 from slack roots. 



(c) Internal Differentiation. The 

 cells of the typical growing point maintain 

 their power of growth and division ; they 

 are termed meristematic cells. All organs 

 composed of such cells have in principle 



the capacity for unlimited growth. Embryonic tissue is found not only 

 at the growing points, but in the secondary meristems (p. 47). 



A portion of the meristematic tissue, or the whole of it in the case 

 of organs of limited growth, becomes transformed into the somatic 

 cells of the permanent tissues ; in these growth and cell division cease, 

 and sooner or later death ensues (p. 309). 



The internal development of an organ commences close behind the 

 growing point and lasts for a longer or shorter time. While the full 

 development of hairs is frequently very rapid, the definite form and 

 structure of the internal tissues is often only completed after the phase of 

 elongation is ended. When secondary growth in thickness takes place 

 there is no termination to the internal development. The development 



FIG. 259. 1, Shoot of Helianthus 

 annuus with the leaves removed 

 and the pith separated from the 

 peripheral tissues by means of a 

 cork -borer. 2, Stalk of the in- 

 florescence of Taraxacum, split 

 longitudinally by two incisions at 

 right angles to one another ; a, 

 just after splitting ; b, after im- 

 mersion in water. 



