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. 1 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 in 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, i.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 (canesccnt), whitish, owing to presence of numerous minute 

 hairs ; pubescent, with short or soft downy hairs ; pilose, with soft distinct 

 hairs ; hirsute, 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. 



