Organization of Leaves. 9 



!<jus utrlculas I shall compare them to that of velvet, which 

 exhibits that appearance merely in consequence of the pro- 

 jection of the threads of silk above the woof. 



Petals with a smooth surface do not exhibit the same 

 phaenomenon, because their exterior utriculae being plane 

 do not reflect the rays of light like the conical utriculje ; 

 and in Howers, the petals of which are velvety above and 

 smooth below, the utriculae are seen to exhibit both these 

 forms. 



The surface of leaves is furnished with a very large quan- 

 tity of small whitish points, which are scarcely apparent to 

 the naked sight, but which may be clearly disting^iished by 

 the help of a good magnifying glass. These points are the 

 cortical glands of Saussure, the viilliary ghnids of Guettard, 

 the evaporating pores of Hedwig, and the cortical pores oi 

 Decandolle. 



Saussure says that the cortical glands are small oval bo- 

 dies, often transparent and at other times opake, surrounded 

 by a fibre or vessel of the same form, and with which se- 

 veral vessels of the cortical reticulation anastomose. At 

 the two extremities of each gland is a very delicate vessel, 

 which traverses in a straight line the interval between the 

 gland and the vessel that surrounds it, with which it ana- 

 stomoses. He considers these glands, on account of their 

 constant position near the surface of the leaf, as organs de- 

 stined for preparing or secreting the peculiar juices which 

 form the matter for transpiration, or for preparing and assi- 

 milating to vegetables the vapours and exhalations which 

 they absorb by their leaves. 



Guettard, without penetrating into the organization of 

 these glands, contents himself with giving them the appel- 

 lation of milliarij, on account of their great number in cer- 

 tain leaves. 



Hedwig first found that these organs are so many small 

 ajiertures, which he calls evaporating pores or conduits. He 

 Hays that each pore is surrounded by a circumference, and 

 that the interval comprehended between the pore and the 

 circumterence is the reservoir of the pore. In regard to thii 

 use of these pores, he lias no doubt that they serve for CA'a- 

 poration or absorption. 



Hceandolle has also given them the name of cortical 

 pores, and seems to have adopted the opinion of Hedwig 

 to regard to the uses of them. 



Seiiebier, in hia Tjeatise on the Physiology of Vegeta- 

 ble.'-', says that he was never able to discover, even with 

 \Lv help of the best mieroacopcs, the porei of Jlcdwig, 



which 



