30 VEGETABLE PHYSIOLOGY 



structures as hollow stems where there is but little 

 substance of tissue, but where they are required to resist 

 lateral bending, the supporting tissue is situated near the 

 periphery of the stem, and the latter is often still further 

 strengthened by being furnished with ridges or flanges. 

 An instance of an almost converse character is afforded by 

 a young root. In its growth, while it must possess sufficient 

 rigidity to enable it to penetrate the soil, it must be capable 

 of frequent bending to enable it to avoid obstacles. This 

 is most advantageously provided for by a central core of 

 strong tissue, surrounded by more succulent material. 

 The transporting tissue of the centre is comparatively 

 little affected by the flexures of the structure, and its 

 function is not interfered with. 



Another kind of differentiation in such a cell-mass as 

 we are dealing with, is the setting apart of particular 

 groups of cells for various metabolic 

 purposes. We have the formation of 

 glandular tissue, of the laticiferous 

 systems, and so on. This differentia- 

 tion may be marked also by the pro- 

 duction of definite organs in the proto- 

 plasts, such as are seen for instance in 

 the case of the chloroplasts of the leaves 

 (fig. 33) and other green parts of plants. 

 FIG. 88. CHLOBOPLASTS The habit of life of a plant again 



EMBEDDED IN THE PttO- 



TOPLASM OF A CELL OF may influence its structure and the 



THE PALISADE TISSUE , -... ,. ,. ., , , 



OF A LEAF. degree of differentiation of its body to a 



very great extent. The great group of the 

 Fungi afford us an illustration of the degradation of structure 

 which accompanies a saprophytic or parasitic habit. Similar 

 instances of degradation are met with among the flowering 

 plants. 



The needs of the cell-mass thus usually lead to the 

 differentiation in its substance of at least four physiologi- 

 cally different regions the tegumentary, the conducting, 

 the supporting, and the metabolic. The latter includes all 



