2(36 BOTANY part i 



of the former as the result of external factors expresses itself also in 

 the internal structure. 



Space will only allow of a short reference to the two factors of liyht and moisture. 

 Sliade-leaves C'-^) exhibit a quite different structure to the leaves of the same species 

 developed in full sunlight. They are thinner, their palisade cells narrow from 

 above downwards leaving wide intercellular spaces, and further, form only a single 

 layer, while several layers may be present in leaves exposed to full light. In 

 etiolated plants the tissues are little differentiated and thickened cells are wanting. 

 The tissues of water-plants ('^) are similarly develoi:)ed ; thickened cells are wanting 

 in the stem, and the vascular bundles are also reduced, while the leaves recall in 

 structure the shade-leaves. With a limited water-supply or witli increased tran- 

 spiration, on the other hand, the vascular bundles are strongly developed, and 

 arrangements appear in the epidermis which have been considered among the means 

 of protection against excessive transpiration. 



Many of these changes due to external factors can be most shortly 

 characterised by terming them " purposive " reactions. The plant 

 forms those tissues which are most useful to it under the particular 

 conditions. 



At suitable places in this section and in the morphological section 

 of this work, the most important facts regarding the function of tissues 

 have been given. It must be mentioned here that G. Haberlandt 

 has classified and named the tissues on the basis of their respective 

 functions. All tissues with the same function are grouped by him in 

 a tissue -system. (The tissue -systems of this physiological anatomy 

 are thus something different from the tissue-systems as considered on 

 p. 110.) Physiological anatomy is often able to show that these systems 

 correspond to the needs of the plant, both in the structure of the 

 elements of the tissue and in their arrangement. This will be shown 

 here fully for the mechanical tissue-system, and only indicated in the 

 case of the other tissues. 



A. The Mechanical Tissue-System (^*) 



Without a certain amount of rigidity the definite form which is 

 essential to the 23erformance of their functions in most plants would 

 be inconceivable. In isolated cells and in growing tissues this rigidity 

 is attained by turgor and tissue tensions. Since, however, turgor and 

 tissue tensions are destroyed by an extreme loss of water, leading to the 

 wilting of the plant, they do not alone confer the necessary rigidity 

 upon plants. We therefore find special tissues, known as the stereome, 

 Avhich have a purely mechanical function. 



How great are the demands made upon the stability of plants will be at once 

 apparent from a consideration of a Rye haulm ; altliough it is composed of hundreds 

 of thousands of small chambers or cells, and has a licight of 1500 mm., it is at its 

 base scarcely 3 mm. in diameter. The thin stems of reeds reach a height of 3000 mm. 

 with a base of only 15 mm. diameter. The height of the reed exceeds by two iiundred 



