128 



DERMAL SYSTEM 



later stage; the Colts-foot (Tussilago Farfara) affords a striking 

 illustration of this temporary condition of hairiness. 



The greatest possible diversity prevails with regard to the detailed 

 structure of the hairy coverings by means of which plants protect 

 themselves against these different climatic influences. The hairs em- 

 ployed for this purpose may be either branched or unbranched, but in 

 either case are generally multicellular. Where they are closely 

 appressed to the surface of the organ, and all point in the same 

 direction, they often produce a silky glistening effect {Convolvulus 

 cneorum, C. nitidus, etc.) ; if on the other hand they are irregularly 

 twisted or curled, or coiled in corkscrew fashion, they form a woolly or 

 felted coat (Gnapkalium Leontopodium, Banksia stellata, etc. etc., Fig. 

 35). A very dense and compact felt results if such twisted hairs 



become mutually entwined, 

 entangled or interwoven. A 

 particularly effective type of 

 felted hairy covering has been 

 described by Goebel 7i as 

 occurring in the genus Es- 

 peletia (Compositae), which 

 inhabits the paramos of Ven- 

 ezuela. Here both sides of 

 the leaf are covered by 

 unbranched hairs which, after 

 rising vertically from the sur- 

 fig. 35. face for some distance, describe 



Coiled and twisted woolly hairs from the abaxial leaf-surface a Very flat Spiral", they then 



of Banksia stellata. 



once more continue in a more 

 or less vertical direction, but soon undergo a second spiral twist, 

 and so on alternately several times in succession. The most 

 interesting point, however, is that the corresponding twisted regions 

 lie approximately at the same level in all the hairs, so that the 

 whole felted covering acquires a regularly stratified structure. This 

 alternation of loose and compact zones still further impedes surface- 

 ventilation, thus affording additional protection against the desiccating 

 action of the violent winds which, as already stated, prevail in the 

 paramos. More or less dense coatings are also formed by the so-called 

 stellate and tufted hairs. Such hairs arise from an initial cell which 

 divides by anticlinal walls into a number of segments ; each one of the 

 latter then develops independently, so that the hair ultimately consists 

 of several branches or rays all inserted upon a common basal portion. 

 Good examples of stellate hairs are found in the Malvaceae and 

 Cistlneae, in spp. of Groton, etc. 



