ANATOMY OF PLANTS. 



197 



smce they often, instead of the right-hned direction, assume 

 a crooked, folded, or winding form ; and the cells or spaces 

 of the cellular texture, which want their sap, are thus placed, 

 by means of peculiarly constructed slits, in immediate com- 

 munication with the external atmosphere. 



310. 



These slits are commonly oval-shaped, and pointed at the 

 extremities, being encompassed by a border, which consists of 

 a granular or glandular mass, and can frequently be torn 

 off ; (Tab. V. Fig. ^.) The partitions of the cellular texture 

 either unite with this border, or they pass round it without 

 touching it. 



The size of these organs is as various as their number. In 

 the Coronariae, where they are largest, their longitudinal dia- 

 meter is from the twelfth to the twentieth part of a geometri- 

 cal line, and their diameter, in the cross direction, is from the 

 twent5"-fourth to the fortieth part. They are exceedingly fine 

 in the most perfect plants, as the Myrteas, Rosaceae, Legu- 

 minosae, and Caryophylleae, Two hundred of them, at least, 

 might lie upon a geometrical line. 



Their number is as various. The smaller they are, the 

 more numerous they usually are. In general, we can count 

 from fifty to two hundred of these slits upon a square line. 



311. 



These organs have some resemblance to the air-vessels of 

 insects, especially when we compare them with the raised 

 pores of the chrysalis of the Sphinx populi ; (Mein. Comment, 

 depart, quibus insect, spirit, ducunt, Tab. II. Fig. 16.) But 

 there is this difference between these two kinds of organs, 

 that the pores of insects always contain the stem of the air- 

 pipes, whilst the slits of plants are in no immediate contact 

 with the spiral vessels. Yet it is very worthy of notice that 

 these two sets of vessels are produced together ; and as the 

 Ferns first shew the spiral vessels, the first symptoms of slita , 

 are also seen in them. 



