THE FUNDAMENTAL TISSUES 135 



series. This situation is in accordance with the marked conserva- 

 tism of roots. The clear boundaries between fibrovascular and 

 fundamental structures are, for example, as well marked in the 

 cycads, conifers, and angiosperms as they are in the case of the 

 lycopods, ferns, and Equisetales. In the conifers and many dicot- 

 yledons the fundamental group of tissues, although well developed 

 in the young condition of the organ, later entirely disappears. 

 The accompanying illustrations (Figs. 98 and 99) of the root in the 

 balsam fir make the truth of this statement sufficiently clear. In 

 the first of the two figures the organ is shown in a young condition, 

 as is evidenced by the slight degree of development of the woody 

 cylinder. A clear boundary separates the outer region of the root 

 from the tissues belonging to the fibrovascular system. This is the 

 endodermis or innermost layer of the fundamental system. The 

 organization of the second figure shows a further degree of develop- 

 ment of the fibrovascular system, while the fundamental tissues, 

 inwardly limited by the endodermis, have begun to shrivel and break 

 away from the surface of the fibrovascular cylinder. The destruc- 

 tion of the jacket of fundamental tissues is the result of the appear- 

 ance of a corky layer immediately within the endodermis; and this, 

 impervious in its nature, cuts off the supply of nutrition to the exter- 

 nal zone. This region of the root consequently dies and flakes off. 

 In the case of the leaf the fundamental tissues are of consider- 

 able importance, since they constitute the green substance or 

 mesophyll of foliar organs. The limits between the stelar or 

 fibrovascular strands in the leaf and the surrounding mesophyll 

 are in general much less well marked than in the root, but on the 

 whole much more clearly indicated than in the stem. In the 

 Pteridophyta in general the endodermis is distinctly devel- 

 oped in the leaf. In the gymnosperms the limits between the 

 fundamental system and the fibrovascular strands are already less 

 obvious, and in the case of the angiosperms a somewhat similar 

 condition is to be observed. The usual situation justifies the 

 summary statement that in foliar organs the fundamental category 

 of tissues is always well developed and is physiologically of great 

 importance, since it subserves the cardinal functions of transpiration 

 and photosynthesis. Further, the morphological limitation of the 

 fundamental system is only less distinct in leaves than it is in roots. 



