TISSUES AND SIMPLE ORGANS. 59 



ical raetaiTiorpliosis which is not well understood). The final pro- 

 tection, however, is afforded bj tlie formation of a layer of cork, 

 which in some cases begins to develop some time before the falh'ng 

 of the leaf ; in other cases it begins later, and permanently supplants 

 the provisional protection. 



The above-mentioned phenomena offered great difficulties to 

 Nageli, who in his theory of natural descent asserts that a stimulus 

 gives rise to an organ. "We ask: 1. What stimulus calls forth the 

 formation of a scissi on -layer ? 2. What stimulus gives rise to the 

 beginnings of scar-tissue formation, even some time before a scar is 

 present? To return to our subject, I will state for the benefit of 

 those who wish to enter more deeply into these relations that the 

 vascular system of leaves (" leaf -trace ") of many growing trees is 

 abscised three times, or even oftener, in the course of the vegetative 

 period ; first by the falling of the leaf, then again a little below the 

 leaf-scar by the above-mentioned scar-tissue formation, and finally 

 still deeper in the interior of the cambium by the growth in thiclr- 

 ness of the stem (this occurs repeatedly among evergreen conifers). 



The necessity for the cork-tissue formation on stems growing in 

 thickness has already been indicated. In only a few instances can 

 the growth of the cuticularized epidermis keep pace with the growth 

 in thickness of the stem ; as a result it is ruptured. From this 

 follows the necessity of a new, somewhat more deeply located, layer 

 of cork to guard against excessive evaporation. The plant behaves, 

 if the expression may be allowed, as if it knew what would happen 

 later. Such "knowing" is, however, excluded : the occurrence of 

 suitable processes is only in obedience to natural laws given by the 

 Creator. Human intelligence is capable of comprehending the 

 teleologioal moment of these, and similar adaptations. The causal- 

 mechanics, the causa efficiens, of the development of cork-tissue is, 

 however, unknown to us ; this is usually the case. 



Since the cambium-ring continues its activity for years, the 

 cork-covering first formed shares the destiny of the epidermis ; it 

 is ruptured, and again a substitute is formed in the interior : 

 that is, other cells situated more and more toward the interior 

 become suberized. 



One of the most useful exercises for the beginner in plant- 

 anatomy is to find the exact location of the first cork-formation in 

 stems and roots. Such investigations teach that in the stems the 

 epidermis itself may give rise to cork-formation {jph in Fig. 36 



