104 



• KNOWLEDGE ♦ 



[Aug. 17, 1883. 



WbitG Alvssum hair, x 70. 



Drosera rotundifolia, 



30. Deutzia scabra, x 70. 



Stinging-nettle hair, x 70. 



Heritiera minor, x 120. 



may have many cells at their base and cellular divisions in 

 the upper part. Round the edge of a sweet-briar leaf are 

 hhort, stumpy tubes ending in a pretty ruby-coloured gland. 

 Rimilarhairs, but less brightly-coloured at their tip?, are abun- 

 dant on the under surface of the leaf, and all are glandular, 

 secreting the sweet scent. Extremely beautiful hairs of 

 this type, but longer, decorate the stems of London 

 pride. They form very striking objects when bril- 

 liantly lit up and magnified about fifty times. The 

 elegant pink heath (Erira titralix) is similarly orna- 

 mented. Glandular hairs may absorb as well as 

 secrete. The remarkable hairs of the Drosera and of 

 other insectivorous plants are illustrations ; but the 

 character of Drosera hairs entitles them to be called 

 tentacles. They have a good deal of true leaf structure 

 in them, and the glandular tips are composed of com- 

 jilicated groups of cells. Botanists reckon them amongst 

 the trichomes, but their whole structure shows how close 

 they are to expansions of the leaf, and not, like the simplest 

 hairs, mere outgrowths of epidermis. They are prey- 

 capturing and digesting organs, much like the tentacles of 

 anemones. The short hairs in the middle of the Drosera 

 leaf can convey impressions to the marginal hairs, causing 

 them to close and hold fast the prey which is attracted and 

 caught in the secretion, which gives the popular and pretty 

 name of Sun Dew to the plant. " If an insect adheres to 

 only a few of the glands of the exterior tentacles, these soon 

 become inflected, and carry their prey to the tentacles next 

 succeeding them inwards ; these then bend inwards, and so 

 onwards until the insect is ultimately carried by a curious 

 sort of rolling movement to the centre of the leaf" (Darwin). 

 Experimenting with other leaves, Darwin found the glan- 

 dular hairs on those of two kinds of saxifrage, of a 

 primula, and a pelargonium would rapidly al)sorb weak 

 solutions of carbonate of ammonia and weak infusions of 

 raw meat. The glands of an erica, a mirabilis, and a nico- 

 tiana did not exhiliit such a nower. With reference to the 



above observations, Darwin remarked* : — " The glandular 

 hairs of ordinary plants have generally been considered by 

 physiologists to serve only as secreting organs ; but we 

 now know that they have the power, at least in some cases, 

 of absorbing both a solution and the vapour of ammonia. 

 As rain-water contains a small percentage of ammonia, and 

 the atmosphere a minute portion of the carbonate, their 

 power can hardly fail to be beneficial, nor can the benefit 

 be quite so insignificant as it might first be thought, for a 

 moderately fine plant of Primula sinennsheess the astonish- 

 ing number of about two millions and a-half of glandular 

 hairs, all of which are able to absorb ammonia brought to 

 them by the rain." 



The bails that form the pappus of thistles and allied 

 plants are sometimes simple, and of others feathered, as 

 in the Carline thistle. On the anther filament of spider- 

 wort {Tradiscantia Virginica) the hairs are like chapleta 

 of beads, and each bead shows the cell rotation of the 

 sap, if put in a drop of water, covered, without squeezing, 

 by thin glass, and magnified three or four hundred times. 

 AH hairs of plants and trees are said to exhibit this sap 

 movement at some period of their lives. 



A shrub, common in gardens, Deutzia Scabra, has an 

 immense number of star-shaped hairs on its leaves, chiefly 

 on the upper surface. These are thickened with an 

 exquisite deposit of silex, and are fine objects under the 

 polariscope and a magnification of from fifty to one hundred 

 times. 



The rays of the larger Deutzia stars are usually four or 

 five ; besides these there are multitudes of smaller star.-i 

 with many more rays. Similar stellate hairs, but not so 

 big, occur on the smaller-leaved Deutzia /jracilis. 



iSlany plants have multiple-rayed stars on their leaves. 

 One of them, Ileritii'.ra minor, a tree found on the coasts of 

 India and Africa, and cultivated in the West Indies, has 



* "Insectivorous Plants," pp. 354-5. 



