XVI FIRST PRINCIPLES 



23. Spiral vessels (Vasa 'spiralia, Lai. ; Trachees, Fr. ; Spiralgefasse, 

 Germ.) consist of elastic tissue twisted spirally into the form of a cylinder, and 

 capable of unrolling. 



24. They are found in the medullary sheath (86), and in all parts that emanate 

 from it in an ascending direction ; viz. the veins of the leaves, and every thing 

 that is a modification of them. 



25. They are not found in any part which is formed in a downward direction ; 

 and are consequently absent from the wood, bark, and root. 



26. The function of the spiral vessels is unknown. 



27. They only exist in plants propagated by the agency of sexes. 



28. Hence the two primary divisions of the vegetable kingdom ; viz., Vasculares, 

 or plants furnished with spiral vessels and propagated by the agency of sexes ; and 

 Cellulares, or plants destitute of spiral vessels and not propagated by the agency 

 of sexes. 



29. Ducts (Fausses trachees, Fr.; Saftrohren, Germ.) are transparent tubes, the 

 sides of which are marked with dots, or bars, or transverse streaks. 



30. Sometimes they have the appearance of spiral vessels, from which they are 

 known by not being capable of unrolling. 



31. Their sides are not perforated by visible holes or pores, except in Coniferae, 

 and Cycadeae, in which perforations are supposed to exist. 



32. They are found among the woody fibre, exist in great abundance in the wood, 

 and their ends are in immediate connexion with the loose cellular tissue occupying 

 the extremities of the fibres of the roots. 



33. Their functions have not been accurately determined. It is probable that 

 they serve for the passage of air. 



34. The mode in which the different forms of tissue are developed is unknown. 



35. There are no other elementary forms of tissue. Air-vessels, Reservoirs of 

 oil, Lenticular glands, Proper vessels, are all either distended intercellular pas- 

 sages, or cavities built up with cellular tissue. 



36. When such caviiies are essential to the existence of a species, they are 

 formed by a regular arrangement of cellular tissue in a definite and unvarying figure ; 

 Ex. Water-plants. When they are not. essential to the existence of" a species, 

 they are mere irregular distensions or lacerations of the tissue ; Ex. Pith of the 

 Walnut Tree. 



. 37. All these forms of tissue are enclosed within a skin called the cuticle. 



38. The Cuticle is an external layer of parenchyma, the cells of which are 

 compressed, and in a firm state of cohesion. 



39. The spaces seen upon the cuticle, when examined by a microscope, represent 

 these cells. 



40. It is, therefore, not a peculiar membrane, but a form of cellular tissue. 



41. It is spread over all parts of plants, except the stigma (345). 



42. The mass of cellular tissue lying beneath the cuticle of the bark is called 

 the epidermis. 



43. The cuticle is often furnished with stomata. 



44. Stomata are oval spaces lying between the sides of the cells, opening into 

 intercellular cavities in the subjacent tissue, and bordered by a rim, the nature of 

 which is not well known. 



45. It is not improbable that this appearance of a rim is due to the juxtaposition 

 of two elastic vesicles, closing up or opening the aperture on which they lie, 

 according to circumstances. 



46. Stomata are found abundantly upon leaves, particularly on the lower surface 

 of those organs ; occasionally upon all parts that are modifications of leaves, espe- 

 cially such as are of a leafy texture ; and on the stem. 



47. Stomata have not been found upon the roots, nor on colourless parasitical 

 plants, nor the submersed parts of plants, nor on cellular plants destitute of ducts ; 

 they are rare, or altogether absent from succulent fruits, and from all parts in a state 

 of anamorphosis. 



48. Any part in which there is an unusual degree of cellular dcvelopemcnt, is said 

 to be in a state of anamorphosis. 



49. The function of stomata is to facilitate evaporation. 



II. COMPOUND ORGANS. 



50. From peculiar combinations of the elementary organs are formed the com- 

 pound organs. 



