28 THE FLOWERING PLANT. 
The last heading to be mentioned regarding the external form 
of stems is that of surface. Not only may a stem be more or less 
ridged, grooved, &c., but also it may possess a more or less com- 
plete clothing of hair-structures, and different names are employed, 
such as silky, hirsute, &c., to indicate the kind of surface produced 
by them.! Absence of hairs is denoted by the term glabrous. 
Hairs on the stem (and leaf) are rarely so simple as root-hairs. 
They are sometimes, however, unicellular, but im this case often 
assume more or less complicated forms. The stems (and leaves) 
of the wallflower, for example, are covered by spindle-shaped 
hairs, attached by their centres, and upon the stems (and leaves) 
of the stock and shepherd’s purse much-branched unicellular 
hairs are present. It is more common, however, to find the hairs 
upon the stem (and leaf) multicellular, and they may then either 
be simple or branched threads, or else more or less complex scales, 
with or without stalks. Glandular hairs are also common, and 
these usually consist of a rounded head, supported upon a short 
stalk. The protoplasm of the head is capable of producing or 
excreting a sticky or oily substance, often of fragrant nature. <A 
thin cross-section of a young pelargonium flower-stalk or vege- 
table marrow stem will, when placed under a low power of the 
microscope, be seen to possess numerous multicellular hairs, 
glandular and non-glandular. 
The surface of the stem may be more or less covered by prickly 
structures, and these are not all of the same nature. Some are 
true hair-structures, ¢.e., they are developed from the epidermis 
only, as in hop, cleaver, and borage. Others again are modified 
stem or leaf structures, e.g., in the sloe and hawthorn. In this 
‘case the terms thorn and spine are best employed. But there 
still remains.a third kind, of which the prickles of the rose and 
bramble are examples. ‘These are not hair structures, since not 
only epidermis but ground-tissue as well helps in their formation. 
Nor are they modified stems or leaves, for they arise in no definite 
order, nor do they contain vascular bundles. The name emer- 
gences has been given to them, but it must be remembered that 
all possible gradations are found between them and _ hair struc- 
tures, on which account some have proposed to group them with 
these last. 
1 The following terms describe the character given to the surface of stem, leaf, 
&c., by hairs :— Hoary (canescent), whitish, owing to presence of numerous ininute 
hairs ; pubescent, with short or soft downy hairs; pilose, with soft distinct 
hairs ; Adrsute, with numerous coarse hairs ; setose (hispid), with bristly hairs ; 
villous, with long soft hairs ; tomentose, with matted hairs; silky, with soft 
straight hairs pressed closely to the surface; woolly, with long crimped and 
matted hairs ; hairy, a general term, applied when the separate hairs are dis- 
tinctly visible ; ciliate, applied to a leaf-margin fringed with hairs, 
i 
