Dust and its Importance to Plant Life. 17 



sides of a Cypripedium leaf, and especially the cylindrical dust- 

 bin of the Lily of the Valley, should be specially mentioned. 



Another interesting arrangement is the sessile leaf-cup, well 

 seen in Campanula bononiensis and Veronica longifolia, where the 

 basal lobes of the leaf blade are folded up, forming a sort of basin 

 or trough. 



Where the leaves are opposite one finds every transition 

 between the Labiate type and the beautiful water troughs of 

 .Silphium perfoliatum and Dipsacus laciniatus. In the Labiates 

 there is usually a stipular ridge, ornamented with hairs; water 

 runs down the vein and petiole-gutters and spills out across and 

 through these hairs. More advanced are the " connate leaf- 

 cups " of Caryophyllacae, Gentianaceae, and especially of 

 Cephalaria tartarica. 



The veins of very many leaves are sunk on the upper surface 

 and the vein-gutters so formed are continued into the petiole; 

 hairs often occur in such a way as to form dams or weirs, against 

 which the dust accumulates. The Ash tvpe described by Kerner 

 van Marilaun is exceedingly common. A verv great number both 

 of pinnate, palmate, and pedate leaves also have excellent rain- 

 grooves down the mid rib and branches ; the bean and carrot are 

 especially beautiful examples of this arrangement, as also 

 Clematis, Helebore, Pjeony, and especially Spiraea aruncus, 

 Male Fern, and Bracken. 



In the Horse Chestnut and Lupine types dust is collected at 

 the top of the petiole, where hairs are present to intercept it. In 

 the Sycamore, Lime, Lotus, and Heuchera types it is also at the 

 base of the lamina just above the insertion of the petiole that one 

 finds collections of dust and straining hairs. 



Many low-growing herbaceous plants, such as Auricula, 

 Funkia, and others, belong to what I have called the Radical 

 type. Everything is in this type at once brought down to earth, 

 and the dust and dirt, etc., accumulates just about the point of 

 origm of the young roots. Some of our common plants, such as 

 Gentiana acaulis, are quite as wonderfully designed for this pur- 

 pose as those Bromelias whose water-cups are continually men- 

 tioned in all books on biologv. 



If one includes such arrangements as these, then it is clear 

 that of the miscellaneous series of plants chosen for examination 



