60 BOTANY PART i 



the layer of cork (Fig. 60). The intercellular spaces open on the one 

 hand to the atmosphere, and on the other are in communication with 

 the ventilating system of the underlying living tissues. 



The lenticels frequently form beneath stomata and at an early stage in the 

 development of the cork. The cork cambium which appears beneath the stoma has 

 radially-running intercellular spaces between its cells (Fig. 60 pi), and forms to the 

 outside complementary cells separated by intercellular spaces (Fig. 60 Z). The 

 lenticel soon breaks through the epidermis. Alternating with the complementary 

 tissue the cork cambium in the lenticels forms layers of more closely-connected 

 suberised and lignified cells (intermediate bands or closing layers). These are 

 developed to close the lenticel during the winter and are again ruptured in the 

 spring. 



3. The Mechanical Tissue System ( 44 ). Without a certain amount 

 of rigidity the definite form which is essential to the performance of 

 their functions in most plants would be inconceivable. In isolated 

 cells and in growing tissues this rigidity is attained by turgor (cf. p. 225) 

 and tissue tensions (cf. p. 286). Since, however, turgor and tissue 

 tensions are destroyed by any great 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, which have a purely mechanical function. These tissues 

 are the SCLERENCHYMA and COLLENCHYMA. 



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

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

 of thousands of small chambers or cells, and has a height 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 hundred times, and that of the Rye haulm by five hundred times, the 

 diameter of the base. In addition, moreover, to the great disproportion between 

 the height and diameter of plants, they often support a heavy weight at the 

 summit ; the Rye straw must sustain the burden of its ear of grain, the slender 

 Palm the heavy and wind-swayed leaves (which in species of Eaphia have a length 

 of 15 m. and a corresponding breadth), while at times the weight of the bunches 

 of fruit has also to be considered. 



In plants, however, the rigid immobility of a building is not required, and they 

 possess instead a wonderful degree of ELASTICITY. The Rye straw bends before 

 the wind, but only to return to its original position when the force of the wind 

 has been expended. The mechanical equipment of plant bodies is peculiar to 

 themselves, but perfectly adapted to their needs. The firm but at the same time 

 elastic material which plants produce is put to the most varied uses by mankind ; 

 the wood forms an easily worked yet sufficiently durable building material, and 

 the bast fibres are used in the manufacture of thread and cordage and textile 

 fabrics (e.g. linen). 



(a) Selerenehyma. This is the typical mechanical tissue of fully- 

 grown parts of the plant and is formed of SCLERENCHYMA CELLS 

 (stone cells) or SCLERENCHYMA FIBRES ("bast fibres"). Both when 

 mature are as a rule dead cells with strongly thickened walls consisting 



