126 LECTURE VI IT. 



From the exceedingly various structure and biological significance of the hairs 

 indicated above, it is self-evident that they present the greatest possible diflferences 

 even in their material constitution. It is therefore hardly possible to say anything 

 general concerning them, except that they are all, in their young states, simple cells, 

 or portions of cells, or aggregates of cells, and that their membrane consists originally 

 of cellulose, and their contents of protoplasm and cell-sap. With the further functional 

 development the most various alterations may take place : the protoplasm and cell- 

 sap may either completely disappear, and the hairs thus represent empty vesicles, as 

 in cotton, and many other woolly hairs; or, on the other hand, the cell-contents 

 are not only preserved, but the protoplasm is even very strongly nourished, and 

 is then distinguished by. circulatory movements and other vital peculiarities, as in 

 the stinging hairs of the Nettle, the stellate hairs of Althcea, the jointed hairs on the 

 stamens of Tradescantia, and in many other cases. According to circumstances, and 

 corresponding to its biological importance, the cell-wall remains thin and pliant, 

 or it becomes thickened, silicified, or stony — a process which may be extended to 

 the tissue at the base of the hair (especially well seen, for instance, in the Gourd plant) ; 

 or in contrast to this, certain layers of the cell-wall of the hair become mucilaginous, 

 and swell up in contact with water, or a balsamic substance (ethereal oil, resin, etc.) 

 is deposited between its fine cuticle and the inner layer of the cell-wall; or, finally, 

 the capitula of the hairs become covered with waxy threads, as on the underside 

 of the leaves of the Fern, Gynmogramme calotnelanos, and others. 



In contrast to the numerous forms of hairs which can be distinguished as organs 

 with definite functions, the frequent occurrence of hairs of which a biological 

 significance is neither perceptible nor probable is of special interest. We find, 

 for example, in some water-plants an uncommonly thick covering of hairs, especially 

 on the young parts of the shoot, e.g. in the Water-lily {Nuphar); while in other water- 

 plants again, with a very similar mode of life, this hairiness is wanting. 



The second of the above systems of tissue are the Vascular bundles, or Fibro- 

 vascular strands. I have already pointed out that the simplest rudimentary forms of 

 these are observable already in some Algae consisting of masses of tissue, as well as in 

 the more strongly developed shoots of Mosses, where they are constituted in general 

 as filiform bundles of thin-walled cells, distinguished from the rest of the tissue 

 by their length. The typical form of vascular bundle, however, is so essential a 

 characteristic of the higher Cryptogams and Phanerogams, that these two subdivisions 

 have been placed from of old, under the name of vascular plants, in contrast to the 

 MuschiecE and Thallophytes as the so-called cellular plants. The typical vascular 

 bundles run in the form of thin filaments, often of considerable length, through the 

 soft fundamental tissue of the roots, shoot-axes, and leaves. In the roots, as 

 already mentioned, there always runs but one strand, which lies in the axis, and 

 with which the strands of the lateral roots are joined. It is however question- 

 able whether these root-strands are not rather to be regarded as composed of two, 

 three, four, or more proper vascular bundles. 



Much more various are the relations in the shoot-axes. Only in some water- 

 plants does a single strand of vascular bundles traverse the interfoliar part up to the 

 growing-point; and with reference to its morphological nature the same probably 

 holds good of it as of the axial root-strands. The vascular bundles of the leaves are 



