1^6 LECTURE X. 



increasing age of such plants, the assimilating leaf-surface enlarges ; and accord- 

 ingly there gradually arises the necessity for a larger and more extended root- 

 system, the parts of which, as well as the shoot-axes, gradually grow in thickness. 

 The thickened roots, stems, and branches present not only the necessary solidity of 

 structure, but they are also essentially adapted to meet the increased requirements 

 of the large leaf-surface. In the first place, it is necessary to convey from the 

 earth to the leaves large quantities of water and dissolved salts : this is attained by 

 the woody body, which is the organ for conducting water, continually increasing in 

 thickness. In addition, large quantities of products of assimilation are elaborated in the 

 crown of foliage ; these must, at least in part, be conveyed to the subterranean roots 

 and lower portions of the stem. Accordingly a so-called secondary cortex (secondary 

 phloem), gradually increasing in thickness, becomes developed from the cambium ; and 

 in this secondary cortex, sieve-tubes and parenchyma form the chief conducting organs. 

 At the same time masses of parenchymatous tissue in the cortex, as well as in the 

 wood, are employed for the storing up of reserve materials. If, however, we consider 

 the distances which the ascending as well as the descending nutritive sap traverse 

 in the roots, stems, and branches, it is evident that the ordinary thin epidermis 

 could not afford sufficient protection against injurious evaporation during this 

 movement. In many cases, therefore, in woody plants, the epidermis becomes further 

 developed. Generally, however, it is replaced by a stronger layer — on the parts still 

 young by typical cork-tissue, which possesses in an enhanced degree the properties of 

 the epidermis ; on older parts by the formation of bark, which in its turn affords 

 protection not merely against prejudicial drying up of the sap-passages in the 

 cortex and wood, but also against mechanical injuries which might occur in the 

 course of time. These remarks may serve to indicate simply the meaning of all 

 the structural arrangements which are the result of the activity of the cambium. 

 In true water plants, which transpire but little, or not at all, the necessity of a 

 vigorous supply of water from the roots is of course entirely wanting, and accord- 

 ingly no subsequent formation of true wood takes place ; consequently the increase 

 in circumference of stem and root is unnecessary; and all those adaptations accom- 

 panying the formation of the secondary cortex, and of the cork and bark of the 

 woody land-plants, are also wanting. Of course nature employs yet other means 

 for the attainment in land plants of a considerable size of the body, and for ensuring 

 a long continuity of life to the roots and shoot-axes. Among the Palms and 

 palm-like Liliaceae, for instance, as well as among the Ferns, plants of large 

 dimensions are found, in which great solidity of the older roots and shoot-axes is 

 necessary ; and where practically the same remarks hold good as to the conduction 

 of the sap. The circumstances here, however, are entirely different, in so far that 

 in such plants the stem supporting the huge crown of leaves soon obtains its 

 final thickness beneath the growing point. The tissues serving as passages for 

 the sap, and as elastic masses, are developed equally from the beginning, without 

 any subsequent enlargement in diameter; and this again is connected with the 

 circumstance that the assimilating crown of foliage, although continually renewed, 

 f3oes not annually increase in size as in true dicotyledonous tr^es. 



I will now, in the first place, attempt to characterise as shortly as possible the 

 tissue formations proceeding from the cambium and caused by the growth in thick- 



