THE STRUCTURE OF PLANTS. 



developed ; and thus the cellular tissue, which 

 forms the great mass of vegetable bodies, is pro- 

 duced in every part of the plant. 



When the cells compos- a. 



ing the tissue only meet with 

 the equable resistance occa- 

 sioned by the presence of 

 the adjacent cells, it is no 

 unusual thing to find them 

 assuming a nearly perfect 

 hexagonal form, or that of 

 the cells of the honey bee, fig. 

 c. But according to pressure, " 



or the resistance they receive, they become more 

 irregular,either elongated, rounded, or compressed . 

 Fig. d exhibits a magnified view 

 of those cells placed contiguous 

 to each other. The walls of the 

 cavities are thin and transpar- 

 ent ; they all communicate with 

 each other either by wide open- 

 ings, or by pores or clefts in the 

 thin walls. Some have supposed 

 that the cells communicate with 

 each other at a point where i 



the walls are interrupted, while others have 

 shown that the communication between the 

 cells takes place only where the pores of their 

 sides are invisible ; thus rendering it probable, 

 that it is by exudation that fluids pass from one 

 cell to another. In the woody parts of trees, the 

 cells are greatly lengthened, so as to form a 

 species of small tubes which are parallel to each 

 other ; their walls are thick and opaque, and 

 often become wholly obliterated* This elongated 

 tissue exists in abundance in vegetables; it is 

 much more common than the regular tissue, and 

 is made up of small tubes which are contracted 

 at different distances. Occasionally they taper 

 towards the extremities. It sometimes happens 

 that the cells of the elongated tissue touch one 

 another only at their widest points, whenever 

 intervals or empty spaces are found between 

 them. According Jo some, these cells contain 

 no Uquid, but are filled with air. The medul- 

 lary rays, to be afterwards described, form another 

 modification of the elongated tissue ; in these 

 the cells are very small, elongated, and placed 

 horizontally, instead of vertically. 



The cellular tissue has very little consistence ; 

 it is easily torn. In many vegetables, especially 

 aquatic plants, there are interspersed around the 

 tissue a number of large holes or lacunae, fiUed 

 with air, which, according to some, are rents or 

 holes in the fragile tissue, while others suppose 

 them regularly formed spaces. Sometimes hairs 

 of a peculiar nature have been found on their 

 inner surface, in the form of tufts or pencils. It 

 is possible to distinguish two species of lacunae ; 

 the one having for an orifice the cuticular pores 

 which communicate with the external air, the 



others having no external communication. The 

 latter exist particularly in plants which want 

 the porous tubes. The use of the cellular tissue 

 is simply to contain and prepare the sap. It is 

 not destined to conduct upwards the unprepared 

 sap, because in the bark and in the pith, both ol 

 which have a structure entirely cellular, the as- 

 cent of the sap is not perceived. There are. 

 however, what have been called sap vessels ir. 

 the cellular texture ; but these, originally, are 

 nothing else but extended cells, which are often 

 stretched to a considerable length. 



The vascular vessels, or sap tubes, are formed 

 of layers of elementary cellular tissue, roUed up 

 in such a way as to form canals or tubes, which 

 are more or less elongated, and placed end on 

 end, and whose partitions are often not to be seen. 

 The walls of these tubes are sometimes pretty 

 thick, shghtly transparent, and perforated with 

 a great number of openings, by means of which 

 they difiuse into the surrounding parts a portion 

 of the air or sap which they contain. These 

 vessels are not continuous from the root to the 

 top of the plant, but they frequently j oin with each 

 other, and at last are changed into areolar tissue. 

 The different kinds of vessels are : simple 

 tubes ; the beaded or moniliform ; the porous 

 vessels ; the slit vessels, or false spirals ; the 

 spiral vessels, and a combination of two or more 

 of the above called mixed vessels. 



Simple tubes. The simple tubes vary in size, 

 but they are the largest of all the vessels, fig. e. 

 They are formed of a thin and entire 

 membrane, without any percep- 

 tible breach of continuity, and 

 are found chiefly in the bark, 

 although they are not confined to 

 it, being met with both in the 

 alburnum or newest formed wood, 

 in the matured wood as well as 

 in the fibres of herbaceous plants. 

 They are particularly conspicuous 

 in the stem and other parts 

 of the different species of SJti- 

 phorbia, and in all plants in general containing 

 thick and resinous juices known by the name 

 of the proper juices, to the ready passage of which 

 their great width of diameter is well adapted. 

 Sometimes they are distinguishable by their 

 colour, which is that of the juices contained in 

 them being white in the Euphorbia, yellow in the 

 Celandine, or scarlet in Piscidia erythrina. In 

 the plant they are united in bundles, but a»re 

 detachable from one another by means of being 

 steeped for a few days in spirit of turpentine, 

 when they become altogether colourless and 

 transparent, because the resinous matter which 

 they contained has been dissolved. They retain 

 their cylindrical form even in their detached 

 state, so that the membrane of which they are 

 composed must be very strong. 



