424 MANUAL OP BOTANY 



In many members of these groups, and in many of the Pteri- 

 dophytes the fibro-vascular system is not so well developed, and 

 we find in such plants the stereome is supplemented by the 

 appearance of bands or sheaths of solerenohyma very variously 

 disposed. There is usually a considerable hypodermal develop- 

 ment, besides masses occurring more deeply in the axis. Some- 

 times the cortical ground tissue contains isolated sclerenchyma- 

 tous bands, often of large size ; frequently the perioycle has 

 similar developments. Sometimes, again, the sclerenchyma, 

 instead of being in isolated strands, forms a continuous ring or 

 . sheath. Many stems contain developments in all three regions 

 which are variously connected together. 



The arrangements of stereome tissue in the leaves are 

 similarly varied. The meristele of the petiole is frequently 

 found to have a large amount of sclerenchyma in its perioycle, 

 and this is continued upwards along the chief axis or axes of the 

 epipodium, gradually thinning out as the margin is approached. 

 In the large flattened leaves of some Monocotyledons, bands of 

 sclerenchyma frequently extend completely across the interior. 

 In some leathery leaves large idioblasts of various form act 

 as struts or trabeculse, reaching from one epidermis to the 

 other. The arrangement of the veins in many leaves has 

 already been alluded to. 



The root usually depends for its rigidity upon the great 

 development of fibro-vascular tissue in the stele. As we have 

 seen, the tissues external to the endodermis are not as a rule 

 long-lived, and hence the bulk of an adult root is stelar in origin. 

 Aquatic roots, like the stems of the same plants, have but little 

 development of specially supporting tissue, but owe their rigidity 

 to the targesoence of their cortex. 



CHAPTER IV. 



THE TRANSPIRATION CURRENT — KOOT PRESSURE — TRANSPIRATION. 



Wb have seen in Chapter II. that in terrestrial plants, so long as 

 circumstances are favourable to the vital activity of the organ- 

 ism, a stream of water is passing from the roots through the 

 axis to the green twigs and leaves, where the greater part of it 

 is evaporated. This current has been caUed the transpiration 

 current, and its rate has been ascertained to be on an average 

 about 100 centimetres per hour. The path by which it passes 



