FORMS OF HAIRS AND PRICKLES. 



base in the form of a ratlially construcletl shield, closely appressed to the surface of the 



leaf, as well as the stellate or tufted hairs proceeding from a single epidermis cell— 



e.g. the Rose-mallow {Alihcsa) — are organs of quite unknown function. On the 



seeds and small fruits of very many Phanerogams are found hairs containing air, 



which appear in part as soft, twisted, woolly hairs (e.g. on the seeds of Cotton), or 



as stiff bristles (seeds of Asclepias Syriaca). They serve, as organs of flight, for the 



distribution of the seeds and small fruits ; and they are especially well developed as the 



so-called Pappus of many Compositce and Valerianecß. Proceeding from the stinging 



hairs, we find, further, a series of stronger outgrowths 



of the epidermis, consisting of hard, lignified tissue, 



and, under the name of prickles, clothing the shoot-axes 



and occasionally the leaf-ribs of many phanerogamous 



plants. Some of these, which serve either for protection 



against the attacks of larger animals or as climbing 



organs, are, like the hairs, mere outgrowths of the 



epidermis — e.g. the prickles of the Bramble; more 



frequently, however, the deeper tissue also takes 



part with the epidermis in the formation of prickles, 



and even vascular bundles may terminate in them. 



Finally may be mentioned the fiat, outspread, often 



relatively large hairs of many Ferns, empty of sap in 



the complete state, which clothe the leaf-stalk ami 



occasionally the leaf-ribs of these plants with a dense 



' ramentaceous ' covering. 



The hair-like structures certain!}- owe their existence 

 and great variety to the circumstance that the\- are 

 able to grow forth unhindered from the free surface 

 of the plant. Hence cells and tissue-bodies which 

 possess exactly the aspect of ordinary hairs are found 

 generally in such places where free space for their 

 formation exists; thus, single cells are developed in 

 the form of hairs in internal tissues which are 

 traversed by large, roomy, intercellular spaces. To 

 the most conspicuous of such objects belong the 

 stellate hairs in the leaf-stalks of the Aymphcracetv, 

 traversed by large intercellular spaces; and those in 

 many tropical AroidecE, the tissue of which indeed 

 obtains, by means of such internal hair-like structures, 

 a tenacious stringy texture. According to the principle 



stated (which regards the epidermis as the place of origin of the hairs less than 

 the space free for the development of long outgrowths), hairs, often of consider- 

 able length, occur even on those \ery simple Algae antl Fungi where a true 

 epidermis, or even indeed a distinct superficial layer, cannot be said to exist : 

 the hairs are in such cases simple outgrowths of the free surflices of flat single- 

 layered masses of tissue, as in the Alga CoJeocJuvle scutala, or they are formed 

 at the apex of a filament consisting of cells. 



Fig. 128.— Part of a longitudinal section 

 of the leaf-stalk of Monstera ddiciosa (an 

 Aioid). p, p parenchyma; j, j a scleren- 

 chynia cell or internal hair in the form uf 

 the letter H 



