212 XVI. THE STEM APEX. 



tissues which may be traced into the meristem of the growing 

 apex, but rather of the mechanical arrangement of the cell- walls, 

 which give to the young tissues the necessary firmness. Very 

 clearly marked in this arrangement are the rectangular relations 

 of the anticlinal walls, i.e., those running perpendicularly to the 

 surface of the apex, with the periclinal, i.e., those parallel to that 

 surface. We can, however, retain the terms dermatogen, peri- 

 blem and plerome, because the arrangement of the layers of cells 

 which we see in Hippuris frequently recurs in the growing apex 

 of Phanerogams, and these terms can therefore be conveniently 

 used for denoting definite regions of the growing apex. From the 

 dermatogen, in fact, amongst Angiosperms, if we exclude the 

 very few exceptions, only the epidermis is derived. The vascular 

 system is, however, not always traceable in its origin to the 

 plerome, but often to the periblem also, as must clearly be the 

 case with cortical leaf -bundles. In the earliest rudiments of 

 leaves we see first, in the outermost layer of the periblem, peri- 

 clinal divisions set up (at /), then follow anticlinal. The der- 

 matogen in the protuberances remains unilamellar ; it divides 

 only by anticlinal walls. In the same way in the development 

 of buds, periclinal and anticlinal divisions take place in the outer 

 layer, but in the dermatogen anticlinal only. The vascular 

 bundles which pass out into the flowers and buds must there- 

 fore necessarily traverse the periblem. 



Substitutes for Hippuris. Instead of Hippuris vulgaris we 

 can use Elodea canadensis, which has a very similar, though 

 longer and more slender growing point, which, with great care, 

 can be actually dissected out with needles under the low power of 

 the microscope. Owing to its great length it projects far beyond 

 the rudiments of its young leaves the leaves which envelop 

 it in the form of a loose bud arising from some distance below 

 the growing apex itself. Species of Myriophyllum will also serve. 



Bud of Euonymus japonicus. We will now study a flat- 

 growing apex, such as occurs in most Phanerogams. As an 

 example, we may take Euonymus japonicus (the Japanese Spindle 

 tree), cultivated as an ornamental shrub in many gardens, which 

 can be examined at any time of the year, and the buds of which 

 cut very well. We first prepare cross- sections, in order to obtain 

 a surface view of the growing apex ; and treat the sections just as 

 we did with Hippuris. With a low power we recognise the 

 growing apex as a flat hump, surrounded by the youngest leaf- 



