44 



PART I. — ORGANOGRAPHY. 



Hairs are also useful in aiding the dispersion of some seeds 

 and fruits. The seeds of the Cotton-plant, the Milkweed, the 

 Dogbane and the Willow-herb, are rendered buoyant by them, 

 so that they are readily scattered by the wind. The fruits of the 

 Desmodiums and of Circaea Lutetiana are covered with hooked 

 hairs (see Fig. 146), by means of which they cling to the fleece 

 of animals and are thus scattered. 



The dense hairy clothing so noticeable on many plants in- 

 habiting dry, arid regions, may serve to temper the energy of the 

 sun's rays by day, and also prevent the excessive radiation of 

 heat from the plant and the chilling of its tissues at night. 



In Ferns and some other flowerless plants, hairs undergo a 

 still more important modification, namely, some of them are 

 developed into sporangia or spore-cases, thus subserving the 

 important function of reproduction. 



Fig. 143. — Unicellular, grandular hair of Sage. 



r ig. 144. — Simple unicellular hair from the leaf of the Cabbage. 



Fig. 145. — Unicellular hair of the Evening Primrose. 



Fig. 146. — Hooked hair from the fruit of Circaea Lutetiana. 



Fig. 147. — Branching unicellular hair of one of the Cruciferae. All magnified. 



Classification of Hairs. — Hairs may conveniently"be classi- 

 fied into unicellular and multicellular forms. The unicellular 

 kinds may be simple, as in Figs. 143, 144 and 145, or they may be 

 branching, as in Figs. 147 and 166. The multicellular forms may 

 consist of a single row of cells placed end to end, as in the monili- 

 form hairs of Tradescantia, Fig. 148, and of Mirabilis Jalapa, 

 Fig. 150; or they may consist of cells which lie in a single plane, 

 as the flattened hairs called chaff that occur on many Ferns, 



