ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 237 



and resins must be regarded as equivalent in function ; they occur 

 constantly intermixed, and a volatile oil frequently passes over into 

 a resin from contact with the air. 



2. Cells with mucilaginous membrane, (a) Mucilage-cells of 

 the inner tissue of the leaf cause transparent dots in the Laurineae 

 and Anonacese. (b) Groups of epidermal cells with mucilaginous 

 inner membrane give this appearance in Stijlogijne (Alyrsinese), 

 Gnidia involucrata (Daphnoidege), and some Sapindacese. The mucilage 

 is in these cases always derived from the cell-wall, which has become 

 thickened at the exj)ense of the cell-cavity. Its physiological function 

 is at present uncertain. 



3. Crystal-containing cells. (a) Cells with clusters of calcium 

 oxalate appear as transparent dots in the leaves of some Euphorbiacege, 

 the Alangieae, Combretacese, some Meliacese, and some Rhamnese; 

 the latter containing also single crystals, (fc) Cells with raphides 

 of calcium oxalate cause transparent dots in the Dioscorese, Smilaceae, 

 Decumaria (Saxifragacese), Ampelidese, Balsaminese, and some Tern- 

 stroemiaceae. 



These deposits of calcium oxalate, which are often accompanied 

 by mucilaginous substances, must also be regarded as secretions, 

 although instances are known in which these crystals are resorbed. 



Spicular cells also occur as transparent dots and streaks, as in 

 Moutabea and Gnetum. 



As regards the systematic value of transparent dots, this can 

 only be taken into account if the term is used in a somewhat enlarged 

 sense, to include not only visible dots, but such also as are concealed 

 in the tissue, and can be made out only by section and under the 

 Microscope. With this meaning of the term, the pressure of raphides- 

 sacs in the leaves is constant in the Dioscoreae, Smilacese, and Tac- 

 cacese, although seldom producing visible transparent dots. The 

 leaves of the Laurine^e are always abundantly perforated either with 

 mucilage- or resin-cells, or with both. The occurrence of cells with 

 mucilaginous membrane in the interior of the leaf has been observed 

 only in the Laurinese and Anonacese. Oil- or resin-cells are constant 

 in the Piperacese and Monimiaceae ; from the latter order they are 

 never absent; in the former their detection failed only in three 

 species of Piper. Internal glands with brown radiately crystalline 

 resin are characteristic of the Myrsineee, and but rarely wanting in 

 them ; they are less common in the Primulaceae. In the Myrtaceee 

 the presence of oil-cavities is characteristic of three of the suborders 

 (Chamgelaucie^, Leptospermeae, and Myrteae), while they are absent 

 from the other two (Barringtonieae and Lecythideae). 



Epinasty of Leaves.* — E. Mer finds that the epinasty of leaves, 

 that is, the tendency of leaves to bend downwards owing to the more 

 rapid growth of the upper surface — is attributable to the following 

 conditions : — (1) It is the result of the development of the palisade- 

 cells of the leaf from the influence of light. (2) Transpiration is not 

 necessary to its development. (3) Neither is assimilation nor the 



* Comptes Kendus, xcv. (1882) pp. 1239-42. 



