PROTECTIVE AURAXGEMEXTS OX THE EPIDERMIS. 319 



protective coverings of great variety. Any plants which are still to retain their 

 activity during the summer on the steppes or in the Mediterranean floral district 

 would succeed very badly if only furnished with the bare foliage of the spring 

 vegetation. If such a plant is to be protected from drying up, its transpiration 

 must be lessened. This is effected by various protective arrangements, but best of 

 all by a thick coating of hair. The papilionaceous plants and species of Orache, 

 above all the immortelles and wormwoods {Helichrysuni, Xeranthemum, Arte- 

 misia), which are still in bloom in the height of the summer and can bear the 

 strongest heat of the sun, are, as a rule, thickly covered with hair, and regions, 

 wliich perhaps only a month before were clothed in fresh green, are now shrouded 

 in dismal gray. With the transition from the wet period of the spring and winter 

 rains to the dryness of midsummer, there is a corresponding gradual transition from 

 the green of the bare, succulent hyacinth leaf to the grey of the rigid felt-covered 

 leaf of the immortelle. 



A peculiar appearance is shown in Mediterranean floral districts by many 

 biennial and perennial plants which one spring give rise to a rosette of leaves close 

 to the soil, and in the spring following to a stem bearing both leaves and blossom, 

 which arises from the centre of the rosette. This rosette formed in the first spring 

 has to live through the dry hot summer, and is therefore covei-ed with felted grey 

 haii's; the stem formed in the second j'ear which gives rise to the blossom, since it 

 is formed during the wet period, has no need of the protective hairs, and is there- 

 fore furnished with green foliage. The Salvia lavandtdcufolia and Scahiosa 

 pulsat'dlokles of Gi'anada, the Hieracium gymnocephalum of Dalmatia, and in the 

 Mediterranean flora the wide-spread Helianthemum Tuber-aria may be mentioned 

 as examples of such plants. Their appearance is so strange that one involuntarily 

 asks whether this green leafy stem really belongs to the grey rosette of leaves, or 

 whether some one has not been playing a joke by putting together the stem and 

 rosette of two different kinds of plants. 



These hair- like structui-es, called "covering hairs", whose function is a pro- 

 tection against excessive exhalation, exhibit a very great variety with regard to 

 form. Notwithstanding this diversity, however, a certain degree of uniformity 

 must not be overlooked, inasmuch as in individual species the same kind of hairs 

 are always present. The coat of hair contributes not a little to the characteristic 

 appearance of the species, and therefore has always been considered of especial value 

 in description and discrimination. As a help to description the older botanists 

 introduced a series of expressions into botanical terminology by which to denote 

 shortly and tersely the most pronounced varieties, and this seems to be the most 

 suitable place for explaining these terms — i.e., the forms of covering hairs which are 

 signified by them. 



First, those covering hairs consisting of a single epidermal cell, which grows out 

 beyond the other epidermal cells, must be distinguished and set apart from those 

 M-liich have become multicellular by the formation of separation walls. 



Unicellular clothing hairs in many cases only project slightly above the surface 



