64 THE MECHANICAL FUNCTIONS. 



of cells thus fortified, and the whole cohering in bundles, so 

 as to present greater resistance to forces tending to displace 

 them in the longitudinal direction than in any other. 



Most of the plants which are included in the Linnean 

 class of Cryptogamia have a structure exclusively composed 

 of cells, as has been already shown in the Fucus vesiculosus. 

 But the greater number of other plants have, in addition to 

 these cells, numerous ducts or vessels, consisting of mem- 

 branous tubes of considerable length, interspersed through- 

 out every part of the system. These tubes exhibit different 

 modifications of structure, more especially with regard to 

 the form of the fibres, or other materials, which adhere 

 to the inner surface of their membranes; and these modifi- 

 cations correspond very exactly with those of the vesicles 

 already described as constituting the simpler forms of vege- 

 table tissue. There can be little doubt, indeed, that the ves- 

 sels of plants take their origin from vesicles, which become 

 elongated by the progress of development in one particular 

 direction; and it is easy to conceive that where the extre- 

 mities of these elongated cells meet, the partitions which se- 

 parate their cavities may become obliterated at the points of 

 junction, so as to unite them into one continuous tube with 

 an uninterrupted interior passage. This view of the forma- 

 tion of the vessels of plants is confirmed by the gradation 

 that may be traced among these various kinds of structures. 

 Elongated cells are often met with applied to each other 

 endwise, as if preparatory to their coalescence into tubes. 

 Sometimes the tapering ends of fusiform cells are joined 

 kterally (as seen in Fig. 12,) so that the partitions which • 

 divide their cavities are oblique. At other times their ends 

 are broader, and admit of their more direct application to 

 each other in the same line, being separated only by mem- 

 branes passing transversely; in which case they present, un- 

 der the microscope, the appearance of a necklace of beads 

 (Fig. 13.) When, by the destruction of these partitions, 

 their cavities become continuous, the tubes they form exhi- 

 bit a series of contractions at certain intervals, marking their 

 origin from separate cells. In this state they have received 



