THE NATURE AND USES OF I'LANTS. 



11 



minute. If an apple, ox other pulpy fruit, be 

 placed under the receiver of an air pump, and 

 tlie receiver exhausted, the air contained in the 

 apple escapes only bythe bursting of the epi- 

 dermis ; hence it has been thought, that the pores 

 are so very minute as to be impermeable even to 

 air. But this conclusion is perhaps too hasty ; 

 the epidermis of the apple may be permeable to 

 air, though not in a state of sudden expan- 

 sion. 



Gaps are empty spaces fonned in the interior 

 of the plant by means of a partial disruption of 

 the membrane forming the tubes or utricles ; 

 they are often placed regularly and symmetri- 

 cally. They would appear to be occasioned by 

 the superabundance of the nutritious juices 

 which their vessels are found sometimes to con- 

 tain, without being able to elaborate, and by which 

 they are ultimately ruptured. They do not 

 occur often, except in plants of a soft and loose 

 texture, such as aquatics, though they are some- 

 times to be met with in woody plants also. In 

 their general aspect, they resemble longitudinal 

 tubes interspersed throughout the cellular tissue 

 or pulp, as may be seen in the stems of ferns ; 

 but in the mare's tail, (equisetum) they assume a 

 regularity of disposition, that seems to indicate 

 something more than merely the accidental rup- 

 ture of the vessels. One gap larger than the 

 rest occupies the centre of the stem, around 

 which a number of smaller gaps are placed in a 

 circular row, which is again encircled with a 

 second row of gaps larger than the last, and al- 

 ternating with them, and forming in their ag- 

 gregate assemblage a sort of symmetrical group. 

 In the leaves of herbaceous plants the gaps are 

 often inteiTupted by transverse diapliragms, 

 formed of a portion of the cellular tissue which 

 still remains entire, as may be seen in the trans- 

 parent structure of the leaves of Typha, and 

 many other plants. Transverse gaps are said to 

 be observable also in the bark of some plants, 

 though very rarely. 



Glands are peculiar organs which are observed 

 on almost every part of a plant, and whose func- 

 tion it is to separate from the general mass of 

 the sap of the plant some particular fluid or 

 substance. In their uses, and even structure, 

 they have a near resemblance to the glands of 

 luiimals. They appear to be fonned of a very 

 delicate cellular tissue, in which a great number 

 of vessels are ramified. But this name has been 

 also given to vesicular bodies, which are often 

 transparent and placed in the substance of organs, 

 and are full of a volatile oil which has been pro- 

 bably secreted in their interior. Their peculiar 

 form and structure are very various ; and hence 

 tliey have been distinguished into several species, 

 'i'hus there are, 1st, Miliary glands. These 

 are very small and superficial. They api)car 

 under the forn^ of small round grains disposed 



in regular series, or scattered without order on 

 all parts of the plant which are exposed to the 

 air. 2d, Vesicular glands. These are small 

 reservoirs full of essential oil, and lodged in the 

 herbaceous integument of vegetables. They are 

 very distinct in the leaves of the myrtle and of the 

 orange, and appear under the aspect of small 

 transparent points when those leaves are placed 

 between the eye and the light. 3d, Globular 

 glands. These have a- spherical form, and ad- 

 here to the epidermis only by a point. They 

 are observed particularly in the laf/iafw. 4th, 

 Utricular glands, or Amptillce. These are filled 

 with a colourless fluid, as in the ice plant. 6th, 

 Papillary glands. They form a species of paps 

 or papillEB, something like the papillie of the 

 tongue. They occur in many of the labiatce. 

 Cth, Lenticular glands. Some of these are borne 

 upon stalks, others sessile, 

 or attached to the plant 

 without any appendage. 

 Many tribes of vegetables, 

 as the mallowsand legumin- 

 ous plants, bear on their 

 jjellicles, or on the disk of 

 their leaves, glands of very 

 various forms. Figs, a be, 

 represent the forms of the 

 simpler glands ; d e sessile 

 glands. 

 Hairs. These are small filaments of greater 

 or less delicacy, found abundantly on vegetables, 

 and which serve for the purpose of absorption 

 and of exlialation. There are few plants desti tute 

 of these haire ; but they are observed chiefly on 

 those which grow on dry situations. In this 

 case, they have been looked upon liy some bo- 

 tanists as serving to multiply and extend the 

 absorbing surfaces of vegetables. Accordingly, 

 they are not found on very succulent plants, 

 such as the thick leaved or aquatic tribes. They 

 appear also, to be in many cases the excretory 

 ducts of many glands, and are thus frequently 

 found inserted on a papillary gland. Thus, in the 

 common stinging nettle, the hairs attached to 

 the gland first pierce the skin, and then conduct 

 the iiTitating fluid into the wound ; for when 

 this fluid is dried up, the prick of the hair no 

 longer produces a painful sensation. Hairs have 

 been divided into the glanduliferous, the excre- 

 tory, and lynnphatic. The first are either im- 

 mediately applied to a gland, or sunnounted by 

 a small peculiar glandular body, as in the white 

 fraxinella ; the second are placed on glands of 

 which they appear to be the excretory ducts des- 

 tined to pour out the secreted fluids, while the 

 third are only a simple prolongation of a corticiU 

 pore. Their forms are various, as the simple- 

 liranched, awl-shaped, bead-shaped : some are 

 hollow and cropped at different places by hori- 

 zontal partitions. Their disposition and existenca 



