90 



MORPHOLOGY OF TISSUES. 



with the parenchymatous cells which adjoin them by such lattice plates, which, 

 although they do not bring the cells into communication, must nevertheless facilitate 

 the interchange of certain constituents by diffusion. 



An illustration of the coalescence of cells is furnished by the vessels of the wood 

 (mostly filled with air) especially those which have bordered pits, as well as the wood- 

 cells with bordered pits {Tracht'ides), of which sufficient has already been said (P^igs. 23- 

 27, pp. 25-27). In the vessels of the secondary wood of Angiosperms formed from short 

 cells with large cavities, the transverse or oblique septa commonly disappear altogether, 

 so that the entire row of cells forms a completely continuous tube. But frequently, 



as in Helianthus^ Sonchus, Cirsium, &c.*, the 

 septa are only partially absorbed, thick ridges 

 remaining, which have a reticulate or lat- 

 ticed form, or, when the septa are very ob- 

 lique, even scalariform. When true wood-vessels 

 form air-conducting tubes in this manner, the 

 separate parts having previously been closed 

 cells, the tubes are also in communication with 

 one another laterally through the open bordered 

 pits already described. In tracheides which are 

 arranged in a prosenchymatous manner, as those 

 of Conifers (Fig. 23, p. 25) and Ferns (Fig. 27, 

 p. 27), this lateral communication is the only one, 

 since in the cells pointed at both ends there are 

 no true septa which could be broken through. It 

 is, on the other hand, doubtful whether the cells 

 out of which annular and spiral vessels are formed 

 (see Fig. 18, p. 23) are always in communica- 

 tion with one another, especially when the spiral 

 cells remain short, as in the ultimate branches 

 of the vascular bundles in the veins of leaves, 

 where they are often considerably enlarged. 

 Preparations in which the structure can be very 

 easily observed may be obtained by boiling very 

 young leaves for some time in potash solution 

 and then placing them in glycerin. Where, 

 however, the spiral vessels are formed at an 

 early period in these organs, attaining subse- 

 quently a considerable length, so that the coils 

 of the spiral thread which w^ere at first very 

 close become vi'idely separated (Fig. 75), it may 

 be assumed that the thin membrane which sepa- 

 rates the contiguous ends of the spiral prosen- 

 chymatous cells becomes ruptured, and thus the 

 cells are placed in communication with one another for considerable lengths. For these 

 and other reasons, it is convenient not to limit the definition of a vessel to cases 

 in which the component cells actually coalesce into a tube. Here, as elsewhere in 

 the different forms of tissue, we find transitional structures ; and the definition of terms 

 must not be founded on a single characteristic selected arbitrarily, but on a general 

 consideration of all the morphological and physiological characters. 



The forms of coalescence now described possess the common physiological function of 

 providing a means for the transport of food-materials, and of promoting and accelerating 



■f 



Fig. 73.— From a very young fibro-vascular bundle 

 of a young petiole oi Scrophularia aqiiatica; part 

 of a spiral vessel surrounded by procambium ; two 

 spirally thickened cells are in prosenchymatous appo- 

 sition ; by the elongation of the petiole the coils of 

 the spiral band, now lying close to one another, are 

 drawn apart ; the spiral band becomes detached from 

 the thin wall which is common to the vessel and to 

 the adjoining cells, and in this way a spiral band is 

 formed capable of unwinding. 



See E. Tangl in Sitzungsber. der kais. Akad. der Wiss. Vienna, May 1871. 



