16 Dust and its Importance to Plant Life. 



the Maidenhair Fern. It is only some more or less glaucous 

 plants and a very few others that appear to make no use of the 

 dust. But it is not safe to decide that every glaucous plant is 

 rain-shy and uninhabited by mites. Gypsophila is a good case 

 in point, for, although at first sight one would think that there is 

 no dust trap of any kind, I found many mites living in the small 

 hollows formed by the connate leaf bases, and the stem was not 

 wax-covered at this part. Thalictrum also has an enlarged base 

 to the leaf, which was swarming with mites. 



Many plants with shiny leaves possess hairs only on the stem 

 or leafstalk. Dust collects either on such hairs or in grooves and 

 wrinkles of the bark, as e.g., in the Common and Portugal Laurel, 

 Rhododendron, Holly, Box, and Ivy, and mites -were found in 

 every case. 



There are several other types in which the leaf blade shows 

 little or no special adaptation to retaining dust, which is how- 

 ever retained by hairs developed on the stem or leaf base. In 

 the Asperula type, the water spills down between the leaves 

 through hairs developed just under the whorl ; in Chickweed and 

 Veronica chamaedrys there is a row of hairs which acts as a water 

 conduit, and which are said to be absorbent. The ligules of 

 Grasses and Ochrea of Polygonaceae often also retain dust. 



One of the commonest types is what I have called " petiole- 

 gutters." The leafstalk is grooved or channelled, and leads 

 water down to hairy buds or grooves on the stems, as is well seen 

 in the Barberry. In the Aconite type the young twigs are nearly 

 in contact with the petiole below them so as to form a very neat 

 dustbin. In Cleyera P'ortunei there is a curious twist of the 

 petiole which results in a small space often crammed with dirt. 



But it is more usual to find a widened out base to the petiole 

 which is developed, in the Umbelliferae, into a vagina, excellently 

 adapted not only to keep any useful material washed in but to 

 form a resting place for insects. In the composite type, the 

 auricles at the base of the, usually, broad and flat petiole and in 

 other plants stipules also strain out of the rain-water any dust 

 which may be brought down the stem. 



There are also amongst the monocotyledons some very neat 

 modifications at the base of the leaf which retain enormous 

 quantities of dust and other refuse. The amplexicaul base of 

 Polygonatum, the widened sheath of Commelina, the folded-up 



