270 BOTANY part i 



together. In accordance with this principle many leaves become plaited or rolled 



(Fig. 217), and so acquire a sufficient rigidity 



^ , without the assistance of any specially developed 



^■^■^■■^^""■^^"'^■^'"^^"^ mechanical tissues. 



In addition to the rigidity of the individual 



-y ^^m,^"^. organs of the plant, their arrangement and 



I ' / f 1 7^'A position are of importance for the stability 



/ I I J '^^ ^'^^ whole organism. The lateral branches, 



V i / which are normally arranged as symmetrically 



X I ^^om^^^ as possible, may be forced by bending them 



Jf (J upwards or downwards to take on an elliptical 



cross-section. In Conifers the "white wood," 



Fio. 217. — Dia»rainniatie transverse i-i,£r j. -j. ^ i^i. 



.. . " ., , f ■ which oners greater resistance to stretching 



sections, to show mt^thon of securing ° ° 



ri.<,'idity by folding and rolling. In foi'ces, and the "red wood," which, being denser, 

 the outspread organ A, to withstand offers greater resistance to compression, are 

 a pressure acting in the direction of formed on the sides which are respectively 



the arrow, only the thickness d is r.f,.«f^i,„,l ^„ „«,,,,„. ,.r,„,i n:^\ nni i. c 



•, ,., L ,. ■ ., ^ , , J r, stretched or coniiu'cssed ('•'). the roots of many 



available, but in the folded organ B . ^ -n 



the thickness D', and in the rolled ti'opical trees are so eccentrically constructed as 

 organ C the thickness D". to form relatively thin plank-like buttresses to 



the trunk. 



B. Other Systems of Tissue {~-') 



In considering physiological anatomy the nature of the mechanical 

 tissue-system can be most readily understood since its elements are 

 differentiated in a very one-sided manner, and for the most jmrt 

 perform the single function of giving strength. There are, however, 

 exceptions. In the wood of Conifers the water-conducting elements 

 or at least the tracheides of the autumn wood also serve as stereome ; 

 in the wood of Dicotyledonous trees typical sclerenchyma cells 

 occasionally serve for storage. 



A " main function " is often not so clearly recognisable in the other 

 tissue-systems distinguished by Haberlandt ; the tissues are suited to 

 perform several functions, and are therefore not so shar^^ly 

 differentiated. In spite of this we may distinguish after Haberlandt 

 the following systems, meristematic tissues, tegumentary system, 

 absorption system, assimilatory system, conducting system, storage 

 system, ventilation system, and also excretory and secretory tissues. 

 The functions of these systems are sufficiently characterised by their 

 respective names ; their arrangement can only be referred to in some 

 examples. The assimilatory system consists of cells containing 

 chlorophyll. Since these can only perform their function in 

 sunlight, we find them developed only in above-ground organs, 

 especially in the leaves, which are the special organs of assimilation. 

 Here the assimilatory cells occupy the whole space from the one 

 epidermis to the other, though their shape is different on the 

 up])er and lower sides of the leaf ; the palisade cells arc adapted 

 to light of greater intensity, the cells of the spongy parenchyma to 



