338 LECTURE XV. 



2. Structure and Properties of Growing Organs. 



In our further study of growth we shall have to deal 

 principally with growing organs which consist of a number of 

 cells; it is important, therefore, that we should first of all 

 form some idea of the structure and properties of such 

 organs. 



The members of plants which exhibit morphological 

 differentiation, stem, leaf, and root, and the body of many 

 Thallophytes, grow at first throughout their whole extent ; 

 but at a later period in their development the capacity for 

 growth is possessed by certain parts only. As a rule the 

 growth in length of a stem, a root, or a leaf, is confined to its 

 apex. But this rule is not without exceptions, for it is not 

 unfrequently the case that a zone of cells near the upper or 

 lower ends of the internodes of stems or near the bases of 

 leaves remains capable of growth ; such zones are termed 

 intercalary zones of growth. The growth in thickness too of 

 stems and roots is confined to layers of cells at a greater or 

 less depth from the surface, which may or may not be con- 

 tinuous so as to form complete zones as seen in transverse 

 sections of the members. 



In the growing apex of a member two regions are to be 

 distinguished: (i) the apical region, or punctum vegetationis, 

 in which growth is relatively slow but cell-division active : 

 (2) a region behind the apex in which growth is rapid, that is, 

 in which the increase in size of the cells is active, but there is 

 little or no cell-division, at least by the formation of trans- 

 verse walls. Behind this, growth, at least in length, has 

 ceased. An estimate of the relative lengths of these regions 

 is afforded by Sachs' observations given in the following 

 table. He marked out the terminal internode of the stem of 

 a plant of Phaseolus multiflorus into twelve lengths, each of 

 3-5 mm., and he determined the amount of elongation of each 

 of these lengths during a period of forty hours. 



