21 8 LECTURE XIII. 



larger and longer; the consequence is that since a long stretch of the root 

 becomes curved and convex on the under side\ the free apex of the root rises 

 high above the level of the water. This movement takes place so rapidly that it can 

 be conveniently followed with the eye. 



A long series of induced movements, with which we shall become familiar more 

 in detail subsequently and among which those of the leaves of Mimosa are best 

 known, are similarly effected by alterations in the turgescence of one side; only these 

 alteradons are not caused by evaporation of the water of the cell-sap. These sensitive 

 phenomena depend upon the remarkable fact that, by simple contact or shaking, 

 the protoplasm of the irritable cells suddenly loses the resistance to filtration w^iich 

 is necessary for turgescence, so that a portion of the cell-sap is driven through the 

 cell-walls of the sensitive organ into neighbouring parts, while the walls, which 

 were previously distended elastically, contract, and thus bring about a shortening of 

 the one side of the sensitive organ, in consequence of which the latter becomes 

 curved concave on this side. 



We have already seen that the rigidity of succulent stems and petioles during 

 growth in length, and often also for some time after its conclusion, is caused by 

 ihe tension of the tissues, depending on the turgescence of the parenchymatous 

 tissue and the opposite pressure of the epidermis often strengthened by collenchyma. 

 The rigidity of the older portions of plants which are traversed by woody 

 sclerenchyma strands, and which no longer grow in length, are produced in 

 another M-ay, however. That a tree stem, or a woody branch, or even an older 

 lignified flowering stalk of a shrub, or the haulm of a grass, is rigid and elastic, 

 depends on quite other causes. In these cases, where lignified masses of tissue 

 are always present in the organ, it is these alone, or with the co-operation of tissue 

 tensions also, which determine the rigidity of the organ. As is well known, a thin, 

 woody Willow twig stripped of its cortex is firm and elastic, and thin rods cut out of 

 the wood of the stem are likewise extremely rigid; and even very thin slips 

 of wood possess this property in a high degree. Here the rigidity by no means 

 depends upon the mutual tensions of layers which are themselves limp ; but upon 

 the fact that the woody tissue is itself rigid, hard, and elastic, much as a metallic 

 rod or a crystal. In addition to the remarkable power of rapidly conducting the 

 water absorbed by imbibition in the substance of the cell-wall, the lignified cells 

 have the office in the vegetable world of enhancing the rigidity of the organs, 

 without the intervention of tissue tensions; and the enormous density of the shell 

 of a Cocoa-nut, or of a Cherry-stone, shows how great may be the solidity of lignified 

 cells under certain circumstances. In these examples, however, it is rather the bulky 

 accumuladon of the solid materials which brings about the solidity of the body in 

 quesdon. In the construction of their flower stems and leaf stalks, on the other 

 hand, only a relatively small quantity of lignified masses of tissue comes into use in 

 the form of very thin strands or layers ; these, however, are so distributed accord- 

 ing to mechanical principles in the organs, that they nevertheless produce a high 



^ The upward curvature of partially drooping radicles laid horizontally on a surface of water 

 here referred to was first observed by Ciesielski, but was regarded as a phenomenon of growth. 1 

 gave the correct explanation in the ' Arbeiten des bot. Inst, in Würzburg,' B. I, pp. 395-4CI. 



