Page:Science vol. 5.djvu/48

 SCIENCE.

��IVot.. v., No. KH.1

��Under llic proleclive sj'stcui, we lenrn llint coi'k and cuticle are a kiod of fat, like tallow; the cork l>eing impermeable to cbauges of tem- perature, anil ao securing slowness of freezing atiil I hawing. The special secretions of ethereal oils liavc also been fonnd by Tvndull to be uiicriil iu preventing the escajw of wnrmlli by night, and over-heating by day.

Tlic iiiecliauicitl system includes bast, wood- cells, and sclerenchyroe; some parts resisting IMilling forces, others resisting pressure. The baat-fibres of many plants surpass iron, and in Bome cases even steel of equal cross-section, in their power of bearing weights. Before winter llie walls of the bast may thicken, becoming coUonchyme, and the walls of cambium- cells may also thicken, as a means in both cases of storing food which is given back in spring. As a general rule, stems are strengthened to resist pressui-e. and roots are fortified against iiyitry liy pulling; and lience the circular ar- rangement of mechanical elements in the one case, and their axial situation in the other. But stems growing in water, and aerial roots, reverse the conditions, and accordingly the stnicture: so that we come to have root-like stems and also stem-like roots. The veniition ofleavesisdirected to protect them from injury, and sometimes to roll them np, and so mini- mize evaporation. Nomeleavcsin arid regions have 'water tissue.' with large reservoirs of water for times of drought.

Root-hairs are subsidiary to the surface of the young rootlets for absorbing watej-, and may occur on undei^round st«ms. They abound most in plants inhabiting dry places and in those which transpire freely; and are absent from Con i ferae (whose transpiration is low) and from marsh-planls (where the water- supply is abundant).

The special assimilating organs are the palisade- cells of the leaves, the sponge-cells being only subsidiary. But the sponge-cells are impoitant for transpiration; and in Iteech- trees the leaves in sunshine have much palisade- tissue, whilst those in shade have most s|>onge- tissne.

On the conducting system (leitung»»yslem) our author makes a wide, and we think a justi- fi.ible. departure from current doctrines. Ac- foi-ding to Sachs, the vessels of |»lant8 contain air. and the wood-cells carry water in the sub- stance of their walls (we understand that Siaohs has given up this view). Ilaberluudt shows that the water in the cell-walls is prob- ably fixed as if crystallized; that the spring wood conducts water rapidly, thougii it is thin- walled T thai «!ilcr lias bet'n hIiowii by Ilohncl

��to be lu the vessels: that in palms aiid tree-ferns there is too little prosencliyme to satisfy Sachs's doctrine; and that Sachs's experiments were defective and wrongly interpreted. The wood- cells are merely mechanical, and the water ascends through the vessels and traohelds.

Though the vessels may contain air-bubblea, they do not communicate with intercellular spaces or with stomata, and the low tension of the air iu them would favor a suctorial rather than a supplying function. They have water both by day and night. The air appeai-s in them when the day is somewhat advance<l; and the alternate bubbles of water and air, like Jamin's lubes, favors Elfving's view of the ascent of the water by ' steps ' (as a writer in Xature names it). The transverse walls of some vessels (os tracheids) support starch, which is too heavy for fluids; and the thin membranes permit the slow passage of water, but stoj) the passage of air. In pitted cells or pitted tracheids the dilfusion surfaces are enlargeil at the pits without the walls being weakened. The wide ducts of rattans afford quick passage for water Ijy diminishing adhe- sion; whilst water-plants have few closed ducts. The long vessels are for through passage of water, and a)>ound in stems; hut tracheids are for local supply, and pre<Iominate in leaves of phanerogams and in some cryptogams.

AnotLier part of the conducting system is the conducting parenchyme of the leaves and stem, including the parencb3-mc of the fibro-vascular bundles, the medullary rays, and the transpai-ciit parenchyme around the bundles of the leaves- Those cells convey or store carbohydrates, a» starch and sugar. Their proximity to the ves- sels indicates osmotic action, by which water and substances in solution pass out and in; the conducting tissue aids in the transmission of water; and the vessels may aid In the trans- ference orstoring of carbohydrates. The con- ducting jiarencliyme of the wood-region joins neighboring inedulliiry rays, the contents of which can pass radially in the stem.

The conducting system for the proteids is the cambiform cells and the sieve-tubes, the [ter- forntions of the latter i>emiitting the transmis- sion of nndissolvcil substances. Milk-ducts share in the functions of the sieve-lubes, reach- ing even to the base of the palisade-ccMs of leaves, and being abundant when [he sieve-

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�� ��The passage of air is by the intei-cellulai spaces. CaiVion dioxide can i;>enetrate the cuti- cle; and l>oth stomata and lenticels can open and close so as to regulate the supply. "The

stomata of plants inhabiting arid regions are

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