\fc02 : ' Vm/THE MONOCOTYLEDONOUS STEM. 



is the Ground or Fundamental Tissue. A clear separation of 

 the ground-tissue into pith and cortex is not generally present 

 with the scattered or isolated arrangement of the bundles. 

 Nevertheless, the whole of the internal tissue in which the 

 vascular bundles lie, and including these bundles themselves, 

 can theoretically be distinguished from the enveloping primary 

 cortex under the name of the central cylinder (or stele), the inter- 

 vascular ground-tissue itself being the conjunctive tissue of the 

 stele. The outermost layer or layers of this stelar parenchyma, 

 not separately distinguishable in the present case, will form 

 the pericycle, which in other instances we shall become more 

 clearly acquainted with. 



Under the microscope with a low power we now look for a 

 part of the section suitable for further study, choosing a vasculai 

 bundle which does not lie too near the periphery, because in this 

 neighbourhood the structure of the bundle is simplified. In all 

 cases it is necessary to note accurately in which direction the 

 periphery of the stem lies, in order that we may know which is 

 the inner and which the outer side of the bundle. 



The vascular bundle which we select may appear somewhat 

 as in the adjoining Fig. 40. We first notice the Sheath (vg), 

 which surrounds the vascular bundle, and has become coloured 

 reddish-brown in the chlorzinc iodine. It consists of strongly 

 thickened and lignified sclerenchyma fibres, and has for that 

 reason stained as noted above. It is more strongly developed 

 on the inner and the outer edge of the vascular bundle, weaker 

 upon its flanks. Working from the inner side of the bundle to- 

 wards the outer, we next see at the inner edge an intercellular 

 passage (/), surrounded by narrow cells, only slightly thickened, 

 which are nevertheless coloured yellow by the chlorzinc iodine. 

 Into this intercellular space projects a ring (a), belonging to an 

 annular vessel, which has been torn by stretching. The inter- 

 cellular passage, also, has arisen from the breaking down of cells. 

 Such a method of development is expressed by the term lysigenous, 

 whereas when the intercellular space arises only by the separa- 

 tion of the elements of a tissue, as in stomata, its origin is 

 schizogenous. This torn vessel, together with some others, the 

 thickening layers of which may perhaps also be seen projecting 

 into the intercellular passage, represents the elements first formed 

 in this part of the vascular bundle, elements which were devel- 

 oped at a time when the parts of the plant with which we are 



