Vegetable Physiology. 383 



be seen in situ by placing delicate petals, such as those of com- 

 mon chickvveed, under the microscope. 



It is well ascertained that these tubular cells with spiral or 

 analogous markings lose their fluid contents at an early period ; 

 they are always found filled with air or gas when observed in a 

 fresh state, except in the very early stages of development : hence 

 the name of ' vessels ' generally applied to them must not be taken 

 as indicating that they convey fluids like the tubular vessels ot 

 animals. They are sometimes called tracheae, as though they 

 represented the respiratory tubes of insects, to which they bear 

 a superficial resemblance. To attribute to them any share in the 

 functions regarded as respiratory in j)lants would be a great 

 assumption in the present state of our knowledge. Perhaps the 

 most plausible view that can be taken of their use is to regard 

 them as constituting a flexible framework or skeleton, combining 

 strength and lightnesSj forming a support to the delicate tubular 

 cells with which they are associated, in which the currents of sap 

 appear to flow. 



In very delicate organs, such as petals, and in very young stems, 

 &c., the veins, ribs, or ' vascular bundles,' consist of spiral-vessels 

 and allied forms, with the elongated sap-tubes. Such structures 

 form the rudiments of the solid fibres which run out into well- 

 developed leaves, and are combined within the stem into the firm 

 structure constituting the wood. A considerable difference exists 

 in the arrangement and mode of development of these structures 

 in the two principal classes of the Flowering plants — those with 

 a one-seed-lobed embrvo, called Monocotyledons (grasses, aspa- 

 ragus, onion, &c.), and tiiose with a two-seed-lobed embryo, 

 called Dicotyledons (bean, turnip, flax, &c.). In the Monoco- 

 tyledons the vascular bundles remain always isolated, as at first, 

 traversing the stem as separate fibres, as may be seen in cutting 

 across a stem of asparagus, or of the white lily (fig. 5) ; in the 

 Dicotyledons the bundles, which are developed in a circle, soon 

 come in contact side by side, and form a tube of solid substance 

 separating the pith from the rind or young bark. The causes 

 of these differences cannot be conveniently explained here ; and 

 we hope to have a better opportunity hereafter, when we carry 

 on our inquiry into the special examination of the structure of 

 the organs of the more important cultivated plants. The young 

 vascular bundles, as we have said, consist of spiral or similar 

 vessels, with a bundle of tubular cells (sap-cells, figs. 5 and 6,c), 

 still in the cambial condition. When fully organized, the greater 

 part of the cambial structure developed in the early stages is 

 converted into wood, or liber-tissue (rf), which results from the 

 thickening of tlie walls of those cells by secondary deposits. 

 These layers of thickening are developed in great numbers in 



