222 The Anatomy of Plants BOOKII 



the structure and development of the lenticels in 1873. 

 Haberlandt wrote on the same subject in 1875. 



Investigation of the outer region of the cortex in many 

 monocotyledonous roots led Olivier to the recognition of 

 the exodermis as a definite layer in 1881. He found further 

 that some roots of this group have a true external peri- 

 derm, especially those of the Aroideae and of some of the 

 Liliaceae. This peculiarity he found shared also by many 

 dicotyledonous roots, those of the Ranunculaceae, Gentia- 

 naceae, and Primulaceae affording examples. 



The recognition of the fact that the epidermis in many 

 cases acts as a reservoir for water was due to Westermaier 

 in 1883. Its power of defence by means of stinging hairs 

 was investigated in 1886 by Haberlandt. 



The structure and distribution of the laticiferous tissues 

 were studied byTrecul from 1862 onwards. The true latici- 

 ferous vessels were first investigated by Hanstein in 1864 

 and by Dippel in 1865, while the laticiferous cells were first 

 distinguished from them in 1872 by David. In 1877 the 

 development of the vessels was described by Schmalhausen, 

 and in 1882 Treub and Schmidt independently determined 

 the laticiferous cells to be living elements containing proto- 

 plasm and nuclei. Scott described the development of the 

 articulated vessels in 1882. 



Researches on the co-ordination of structure and func- 

 tion in leaves were conducted by Stahl in 1880 and 

 by Pick in 1882. The former showed the adaptation 

 of the palisade tissue to high, and that of the spongy 

 mesophyll to low, intensities of light. Pick pointed out 

 further that the length of the palisade cells varies with 

 the intensity of the incident rays, and with the inclination 

 of the long axes of the cells to the direction of the light. 

 In this connexion he showed that there is a different 

 organization of the chlorophyll-containing tissues on the 

 north and the south sides of photosynthetic stems. 



