ABSTRACTS OF FOUR LECTURES TO STUDENTS. 



517 



Moreover, the anatomical structure follows suit in that the tissue is alike 

 on both sides, whereas, in horizontally-growing leaves, the tissues of the 

 upper half are quite different from those of the lower. 



In some leaves, as of Alstrcemeria, the blade is reversed in position as 

 the petiole is twisted. This is perhaps due to an effort to increase the 

 strength to support the blade. In such cases the tissues change places, 

 so to say : those normally characteristic of the upper and lower sides now 

 assume those of the lower and upper respectively, showing how readily 

 Nature can adopt whatever structure is most suitable. 



Vegetation may be subjected to great drought, as in the deserts, 

 karroos, and veldts of Africa, where there is an insufficient supply of 

 water to develop large leaves. Under these conditions plants of many 

 different families have a "Heath-like " foliage, or that of the Cypress or 

 Juniper. On the other hand, many may become thick for the storage of 

 water, as Mesembryanthemum, Sedum, Crassula, Sec. 



Leaves of aquatic plants have also their peculiarities. Then, when 

 they float they often acquire an orbicular form, as of Water-lilies, 

 Villarsia, Hydrocharis, &c. 



A mathematician * calculated what ought to be the best form of a 

 blade when floating, to resist the strain of running water so as to avoid 

 injury by tearing, and he found it to be that which Nature has adopted 

 in Water-lilies, &c. 



On the other hand, the submerged leaves are usually finely divided. 

 Such occur in the Water Crowfoot, Cabomba, Water Milfoil, Water 

 Violet, and many others. The inference is that such a form is attri- 

 butable to the water. Experiments have proved that the excess of water by 

 which the protoplasm is saturated can be drawn out through osmotic 

 action, by rendering the water in which the plant grew of a greater 

 density by means of salts, &c. Under these conditions the protoplasm 

 of the stem was able to make the complete form of leaf under water as it 

 does in the air.t 



Compound or Many-bladed Leaves. — These are derived from 

 simple leaves by the gradual separation of portions, i.e. metaphorically 

 speaking, because they really develop freely ; but as intermediate con- 

 ditions are often found, so it is convenient to describe them thus. 



If a Blackberry bush be searched, simple leaves occur with inflores- 

 cences ; but below may be seen a simple but lobed blade. In other 

 leaves the two lobes are quite free, and so a compound or " ternate " leaf 

 results, of three leaflets. The two lower leaflets may be found lobed, and 

 then a completely formed 5-lobed leaf results. The Cinquefoil (Potentilla 

 verna) may have both simple leaves of one blade and 3-, 5-, and 7-bladed 

 leaves, all on the same plant. 



If a vigorous shoot of the Snowberry be examined, it appears to 

 explain the meaning of compound leaves. It will be found to have small 

 oval leaves at the base, and again at the end ; while the leaves in the 

 middle of the shoot are very much larger and lobed. 



At the beginning and end of the season, vegetative vigour was weak ; 



* " A Theory of the Forms of Floating Leaves in certain Plants," by W. P. Hiern, 

 M.A., Proc. Cam. Phil. Soc, Oct. 1872. 

 t Hot. Gaz. xxxiv., Aug. 1902, p. 93. 



