LATICIFEROUS AND VESICULAR VESSELS, ETC. IO9 



rays whicli lie between the cambium-layers of each pair of adjoining bundles (Fig. 92, 

 C, cb). A bridge of secondary meristem is thus, as it were, established between the 

 cambium layers of the bundles ; and thus a closed ring of generating tissue is again 

 formed, which also occasions the thickening of the portion of the stem, and may hence 

 be termed a ' Thickening-ring ' ; but its origin is somewhat different to that in Dracaena 

 and its allies. In them the thickening-ring has its origin entirely in the secondary 

 meristem which was formed from the fundamental tissue, and the newly formed fibro- 

 vascular bundles lie in the thickening-ring; here, on the other hand, the thickening- 

 ring (C, cb) consists of cambium which lies in the vascular bundles, and of secondary 

 meristem which proceeds from the fundamental tissue. Here, therefore, the 

 thickening-ring passes through the fibro-vascular bundles ; but the fundamental tissue 

 which generating the parts required to complete the ring between the bundles has 

 itself only shortly before been formed from a generating tissue. Subsequently the 

 cambium of the bundles constantly produces new xylem, the meristem between them 

 does the same, and thus is formed a closed ring of xylem {i.e. a hollow cylinder), which 

 continually increases in thickness; simultaneously the same thickening-tissue forms 

 constantly towards the outside new layers of phloem. As soon as this takes place, all 

 perceptible distinction ceases between the original cambium of the bundles and the in- 

 termediate secondary meristem ; or a closed cambium-ring is formed. The fibro-vascular 

 masses which are now constantly formed accumulate greatly, while the original funda- 

 mental tissue diminishes more and more in mass. By the increase in size of the 

 fibro-vascular substance in the segment of the stem, the epidermis and the cortical 

 parenchyma become passively extended ; their cells grow rapidly in a tangential direc- 

 tion ; but their original form is again restored as they become divided by radial walls ; 

 and thus also division is subsequently brought about in the whole portion of the original 

 fundamental tissue and the epidermis, by the processes which take place in the fibro- 

 vascular substance. Fig. 56 (p. 69) represents these phenomena in the thickened hypo- 

 cotyledonary segment of the stem of the sunflower ; the figure however is equally avail- 

 able for Ricinus. 



Skct. 18. Laticiferous and Vesicular Vessels, Sap-conducting Inter- 

 cellular Spaces, Glands. — Like oilier forms of cells and tissues, those of which 

 we are speaking occur both in the fundamental tissue and in the fibro-vascular 

 bundles, and even in the epidermal system ; and by a strict carrying out of the 

 morphology of tissues these forms would also be considered as constituents of the 

 three systems. If we nevertheless treat them both separately and together, the object 

 is to place more conspicuously in the foreground their prominent physiological pecu- 

 liarities. They show manifold transitions both to the forms of tissue of the system 

 within which they lie, and among one another. The more simple vesicular vessels 

 which occur especially in the parenchyma of the fundamental tissue of many Mono- 

 cotyledons differ only by the greater length of the cells and by their union in rows 

 from the surrounding parenchyma-cells ; when more mature the cells of these rows 

 coalesce ; the septa become absorbed ; and thus longer tubes, mostly placed near 

 the epidermis, are formed. From these to the true laticiferous vessels is only a 

 step. They are also the result of the coalescence of rectilinear or branched anasto- 

 mosing rows of cells. These canals, filled with milky sap, lie abundantly in the 

 phloem-portion of the bundles, and accompany them through all parts of the plant, 

 forming in it a continuous system. They occur also in xylem {e. g. Carica), where, 

 originating from the coalescence of parenchyma-cells, they form an envelope round 

 the vessels, and even penetrate into the cortex by means of the medullary rays ; in 

 other cases again they form part of the fundamental tissue of the pith or cortex. 



