196 LATICIFEROVS TUBES, 



Euphorbia Lathyris : they are thick transverse cellulose plates in the main tubes. 

 They perhaps show, as above stated, that new branches and continuations of the 

 tubes coming from below arise from cells of the meristem, which coalesce with them. 

 Perhaps they are subsequently formed structures in the originally continuous sac. 



Observations on the growing plant, from the stage of germination onwards, 

 having constantly proved the existence of tubes which extend continuously into the 

 extreme meristem, there was reason for supposing that these arise in the embryo in 

 small number only, and that, once formed, they grow on with the plant in such a 

 way that the whole system of tubes of the stock, exclusive of the layers of secondary 

 cortex, is derived from their elongation and branching. In order to test this suppo- 

 sition, Herr J. Schmalhausen undertook in the laboratory at Strassburg an investiga- 

 tion of the development of tissues in the embryo of species of Euphorbia (E. Lathyris, 

 Myrsinites, Lagascse). I append the summary of his results up to the present time word 

 for word, as it was given to me, with the remark that what is said on the coalescence 

 of branches has, as above indicated, always appeared doubtful to me. 



' The first initials of the laticiferous tubes appear at a very early stage of develop- 

 ment of the enabryo of Euphorbia. At that moment, when the cotyledons begin to make 

 themselves prominent, there are single cells, lying approximately in the same transverse 

 section of the embryo, which first distinguish themselves from those surrounding 

 them by a peculiar refractive property of the cell-walls, which gives them the 

 appearance of being swollen. At this time the plerome cylinder is clearly marked 

 off at the apex of the root from the three-layered mantle of periblem and dermatogen 

 by a boundary line, which appears sharply in optical section ; while in the upper 

 cotyledonary portion of the embryo no arrangement in layers is visible. Where the limit 

 between the plerome cylinder and cortex of the end of the root ceases at the upper 

 end of the latter, that is in the part belonging to the cotyledonary octants, the cells 

 in question may be seen so placed that the line separating the plerome cylinder from 

 the cortex leads up to the base of these cells. These original cells of the laticiferous 

 tubes extend greatly at first in different directions, so that they can now be easily 

 recognised by their great size. As the embryo continues to grow the cells increase 

 in length, and, thrusting their upper and lower ends between the cells of the sur- 

 rounding tissue, they put out processes upwards into the cotyledons, and downwards 

 into the end of the root. Besides this, lateral processes are formed, which compose 

 a felt in the node of the embryo, and surround its growing-point. Thus the sacs of 

 the embryo are formed, not by coalescence of cells, but by apical growth of the 

 processes of the original cells, which force themselves between the cells of the 

 embryo : where two processes meet with their ends, the wall separating them is 

 often absorbed, and a coalescence of the sacs takes place, as is the case also in the 

 nodes and between the branches of the main tubes in young leaves (or cotyledons): 

 this occasionally occurs also in the apex of the root. 



' The best proof that the sacs have an independent apical growth, and do not 

 arise by coalescence of cells, is obtained at the apex of the root, where they have a 

 direct course. The processes of the original cells, which grow into the apex of the 

 root, are arranged approximately in two concentric layers : one series of them 

 belongs to the plerome cylinder— they permeate a layer of cells of the apex of tlie 

 root, which is subsequently found to be within the layer of endodermis ; the others 



