358 A. B. REXDLE ON THE USE OF MICROSCOPICAL CHARACTERS 



characteristic odour to the secretion which is developed in the 

 hair-cell between the cuticle and the outer wall, the secretion 

 being set free by rupture of the cuticle. 



In the interior of the tissue the secretion may be formed within 

 special cells, which may be very long and often branched, as those 

 which contain the milky juice or latex in the spurges and other 

 members of the Euphorbia family, or may consist of short elements 

 placed end to end and perforated, forming vessels, as in the latex- 

 containing tissue of dandelion, chicory and other members of the 

 family Compositae ; or canals may be formed by the separation 

 of rows of cells, leaving a long narrow intercellular space, as in the 

 resin-containing canals of coniferous plants. There is a remarkable 

 external resemblance in the habit of the cactus-like Euphorbias, 

 which are a feature of the arid districts of tropical Africa, and the 

 Cacti which are associated with similar conditions in the deserts 

 of the Southern United States of America. But the presence of 

 the latex-containing cells of the Euphorbias supplies a distinctive 

 anatomical character. The number and position of the resin- 

 canals which are found in the stem and leaves of many conifers 

 are useful characters in the distinction of the species. 



Many of these secretions are, from the point of view of the 

 plant, of the nature of waste-products. Owing to the fact that 

 plants do not perspire and have no kidneys, other ways of getting 

 rid of waste-products of metabolism have to be devised. These 

 products are set aside outside the sphere of metabolic activity, as 

 in the secretory vessels already mentioned, or are deposited in th^ 

 bark which becomes gradually exfoliated, or in the leaves which 

 are ultimately thrown off. An interesting example, which also 

 affords characters of use to the systematist, is found in the Cysto- 

 litlis. These consist of local growths of the cellulose cell-wall 

 which become permeated with minute particles of carbonate of 

 lime. If treated under the microscope with a dilute acid the 

 carbonate is dissolved and decomposed, and we may watch the- 

 tiny bubbles of carbonic acid gas which are given off in the process. 

 The matrix remains as a delicate cellulose skeleton showing well- 

 marked stratification. Cystoliths are very common in two widely 

 separated families, Urticaceae (the nettle family) and Acanthaceae. 

 They were noticed in the former by Meyen in 1839 in the leaf of 

 the well-known India-rubber plant, Ficus elasiica. They occur 

 generally in the large genus Ficus (the figs), varying in form and 



