220 PLANT ANATOMY. 
very sharp contrast to the zones which surround the large ves- 
sels, so that the varieties of wood just mentioned and others 
derived from tropical trees appear as if they had annual rings. 
A transverse section, however, soon teaches that these bands of 
parenchyma do not form connected rings, and that they also differ 
in other respects essentially from the annual rings. A longitudinal 
section shows still more distinctly that they consist of parenchy- 
matous tissue. They are designated as apparent rings.’ 
Such groups of wood-parenchyma never occur in the vascular 
portions of monocotyledons and in the young wood of dicotyle- 
dons, but they are not rare in the older wood, the general struc- 
ture of which may here be considered. 
The most marked distinction between the monocotyledonous 
and the dicotyledonous stem depends upon the development of 
a cambium in the latter (Figs. 129, 138). 
The cambium (cambium ring, thickening ring) belongs to 
the formative tissues, and effects by its activity the growth in 
thickness. Consequently, where a cambium is wanting, as in 
the monocotyledons, a (secondary) growth in thickness is also 
excluded, The stems, therefore, remain slender and thin (Palms, 
Bambusa*). The isolated bundles found in monocotyledons 
become initially formed as such already at the growing point, 
and no merismatic layer is maintained between the vascular and 
sieve portions (compare Fig. 133). 
It is otherwise with the dicotyledons and the gymnosperms. 
Here there appears between the interior, vascular portion, and 
the exterior, sieve portion, a layer consisting of very thin-walled,+ 
? The fine transverse undulations or horizontal stripings which are 
observed upon tangential sections of many woods (Picrasma excelsa, 
Pterocarpus santalinus, Guaiacum, Ceesalpinia, Diospyros, and others) 
proceed, according to Von Héhnel (Berichte d. deutsch. bot. Ges., ii., 3), 
from the horizontal rows of equally large medullary rays or from a trabec- 
ular arrangement of the pits of the tracheids (Tamarindus), or from both 
Causes at the same time. 
_ * Only some tree-like Liliaceze (Aloé, Draccena, Yucca) possess & SeC- 
ondary growth in thickness. 
_ *In drugs the cambium cells are, therefore, either torn, or at least 
_ greatly distorted and bent. Upon a transverse section of the stems and 
