190 



KNOWLEDGE * 



[March 6, 1885. 



He had already suggested one consideration which in 

 certain cases determined the width of leaves, but there 

 were others in which it was due to other causes, one 

 being tlie attitude of the leaf itself. In many 

 genera with broad and narrow-leaved species — Drosera and 

 Plantago, for instance — the broad leaves formed a horizantal 

 rosette, while the narrow ones were raised upwards. 

 Fleshy leaves were princi])ally found in hot and dry 

 countries, where this peculiarity had the advantage of 

 oflFering a smaller surface, and therefore exposing the plant 

 less to the loss of water liy evaporation. 



Sir John then passed to aquatic plants, many of which 

 have two kinds of leaves : one more or less roundtd, which 

 floats on the surface, and others cut up into narrow fila- 

 ments which remain below. The latter thus present a 

 greater extent of surface. In air, however, such leaves 

 would "be unable to support even their own weight, much 

 less to I'esist any force such as that of the wind. In per- 

 fectly still air, however, for the same reason, fiuely-divided 

 leaves may be an advantage, while, in comparatively 

 exposed situations more compact leaves may be more suit- 

 able. It was pointed out that finely-cut leaves are common 

 among low herbs, and that some families which among the 

 low and herb-like species have such leaves, in shrubby or 

 ligneous ones have leaves more or less like those of the 

 laurel or beech. 



An interesting part of the subject is connected with the 

 light thrown by the leaves of seedlings. Thus, the furze 

 has at first trifoliate leaves, which gradually pass into 

 spines. This shows that the furze is descended from 

 ancestors which had trifoliate leaves, as so many of its 

 congeners have now. Similarly, in some species, which, 

 when mature, have palmate leaves, those of the seedling are 

 heart-shaped. He thought that, perhaps, in all cases the 

 palmate form was derived from the heart-shaped, and that 

 when in any genus we find heart-shaped and lobed leaves, 

 the former may represent the earlier or ancestral condition. 

 He then pointed out that, if there was some definite form 

 told ofT fir each species, then surely a similar rule ought to 

 hold good for each genus. The species of a genus might 

 ■well differ more from one another than the varieties of any 

 particular species ; the generic type might be, so to say, 

 less closely limited ; but still there ought to be some type 

 characteri^tic of the genus. He took, then, one genus — 

 that of Senccio (the groundsel). 



Now,? in addition to Senecios more or less resembling 

 the common groundsel, there were species with leaves like 

 the daisy, bushy .species with leaves like the privet and the 

 box, small trees with leaves like the laurel and the poplar, 

 climbing species like the tamus and bryony. In fact, the 

 list is a very long one, and shows that there is no definite 

 type of leaf, but that the form in the various species 

 depends on the condition of the species. From these and 

 other considerations he concluded that the forms of leaves 

 did not depend on any inheritant tendency, but to the 

 structure and organisation, the habits and requirements of 

 the plant. Of course, it might be that the present form 

 had reference to former and not to present conditions. 

 This rendered the problem all the more complex and 

 difficult. 



The lecture was illustrated by numerous diagrams and 

 specimens, and Sir John concluded by saying the subject 

 presented a very wide and interesting field of study, for, if 

 he were correct in his contention, every one of the almost 

 infinite forms of leaves must have some cause and expla- 

 nation. 



The United States Government has issued a series of 10 maps 

 illustrating Prof. Sargent's report on North American forest-trees. 



PLEASANT HOURS WITH THE 



MICEOSCOPE. 



By Henry J. Slack, F.G.S., F.E.M.S. 



rriHE stomata of plants are interesting objects for the 

 X microscope, and the leaf cuticles containing them are 

 often finely displayed with polarised light. Stomata, as 

 their name implies, are little mouths, and their function is 

 to facilitate the interchange of gases between the plant-cells 

 and the external air. They are most abundant where 

 respiration and digestion are most actively carried on, which 

 is in green leaves. They are not found in true roots, and 

 are rare in the submerged parts of water-plants. They are 

 not entirely absent from the petals and carpels of flowering 

 plants, and in the castor-oil plant (Ricintis) occur in the 

 interior cavity of the ovary. Their number in a square 

 inch varies greatly, as may be seen by comparing the supply 

 in a few common plants. The under side of the leaves is 

 the place to look for them, as in a great many cases there 

 are none on the upper side. 



Many greenhouse plants owe the beauty of the underside 

 of their leaves to the arrangement of the stomata. This is 

 the case with some of the Begonias ; and if half-a-dozen 

 varieties are examined, a great diSerence may be seen in 

 the number of the stomata per square inch, and in the 

 patterns they form. In B. metallica, so called from the 

 metallic lustre of its rich greenand-red-veined leaves, a 

 hand-magnifier shows a vast number of tiny rosettes con- 

 taining the stomata. A small jiiece of the leaf cut from 

 between the veins should be placed on a glass slide — 

 moistened, to make it adhere — with the under-side upper- 

 most, and viewed under the microscope with a half-inch 

 power and dark ground illumination. It will then 

 be seen that the little rosettes contain groups of 

 two, three, or four cells, surrounded by plain cells, as 

 shown in the annexed sketch. Taking a bit of leaf from 

 one of the commoner sorts, with large white bands varie- 

 gating the green tint, the stomata are found more numerous 

 and thicker together, but not arranged in a definite pattern. 

 A thick, fleshy-leaved species, red underneath, supplies a 

 very pretty object, though not equal to B. metallica ; but 

 where there is a collection it is worth while to look through 

 them all. 



X5iO 



Stomata of Begonia Metallica. 



In flowering plants the stomata are generally oval forma- 

 tions, in the midst of which a couple of half-moon-shaped 

 thin membranes approach, but do not quite touch each 

 other with their straight sides. These little membranes or 

 thin cells are most often just below the epidermis, and 

 they leave a slit-like orifice as a little open mouth. 



Plant respiration is like that of the animals. Atmo- 



