36 



SCIENCE 



[Vol. V., No. 101. 



Under the protective system, we learn that 

 cork and cuticle are a kind of fat, like tallow ; 

 the cork being impermeable to changes of tem- 

 perature, and so securing slowness of freezing 

 and thawing. The special secretions of ethereal 

 oils have also been found by Tyndall to be 

 useful in preventing the escape of warmth by 

 night, and over-heating by day. 



The mechanical system includes bast, wood- 

 cells, and sclerenchyme ; some parts resisting 

 pulling forces, others resisting pressure. The 

 bast-fibres of man}' plants surpass iron, and in 

 some cases even steel of equal cross-section, in 

 their power of bearing weights. Before winter 

 the walls of the bast may thicken, becoming 

 collenchyme, 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 pressure, and roots are fortified against 

 injury by pulling ; and hence 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 

 structure ; so that we come to have root-like 

 stems and also stem-like roots. The venation 

 of leaves is directed to protect them from injury, 

 and sometimes to roll them up, and so mini- 

 mize evaporation. Some leaves in 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 water, and 

 may occur on underground stems. They 

 abound most in plants inhabiting dry places 

 and in those which transpire freely ; and are 

 absent from Coniferae (whose transpiration is 

 low) and from marsh-plants (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 important for transpiration ; and in beech- 

 trees the leaves in sunshine have much palisade- 

 tissue, whilst those in shade have most sponge-, 

 tissue. 



On the conducting system (leitungssystem) 

 our author makes a wide, and we think a justi- 

 fiable, departure from current doctrines. Ac- 

 cording to Sachs, the vessels of plants contain 

 air, and the wood-cells cany water in the sub- 

 stance of their walls (we understand that 

 Sachs has given up this view). Haberlandt 

 shows that the water in the cell-walls is prob- 

 ably fixed as if crystallized ; that the spring 

 wood conducts water rapidly, though it is thin- 

 walled : that water has been shown bv Hohnel 



to be in the vessels ; that in palms and tree-ferns 

 there is too little prosenchyine 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 tracheids. 



Though the vessels ma}* contain air-bubbles, 

 they do not communicate with intercellular 

 spaces or with stomata, and the low tension of 

 the air in them would favor a suctorial rather 

 than a supplying function. They have water 

 both by da}* and night. The air appears in 

 them when the day is somewhat advanced ; 

 and the alternate bubbles of water and air, like 

 Jamin's tubes, favors Elfving's view of the 

 ascent of the water by ' steps ' (as a writer in 

 Nature names it). The transverse walls of 

 some vessels (as tracheids) support starch, 

 which is too heavy for fluids ; and the thin 

 membranes permit the slow passage of water, 

 but stop the passage of air. In pitted cells 

 or pitted tracheids the diffusion surfaces are 

 enlarged at the pits without the walls being 

 weakened. The wide ducts of rattans afford 

 quick passage for water by diminishing adhe- 

 sion ; whilst water-plants have few closed ducts. 

 The long vessels are for through passage of 

 water, and abound in stems ; but tracheids are 

 for local supply, and predominate in leaves of 

 phanerogams and in some cryptogams. 



Another part of the conducting system is the 

 conducting parenchyme of the leaves, and stem, 

 including the parenchyme of the fibro-vascular 

 bundles, the medullary rays, and the transparent 

 parenchyme around the bundles of the leaves. 

 These cells convey or store carbohydrates, as 

 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 or storing of carbohydrates. The con- 

 ducting parenchyme of the wood-region joins 

 neighboring medullary rays, the contents of 

 which can pass radially in the stem. 



The conducting system for the proteicls is the 

 cambiform cells and the sieve-tubes, the per- 

 forations of the latter permitting the transmis- 

 sion of undissolved substances. Milk-ducts 

 share in the functions of the sieve-tubes, reach- 

 ing even to the base of the palisade-cells of 

 leaves, and .being abundant when the sieve- 

 tubes are few. 



The passage of air is by the intercellular 

 spaces. Carbon dioxide can penetrate the cuti- 

 cle ; and both stomata and lenticels can open 

 and close so as to regulate the supply. The 

 stomata of plants inhabiting arid regions are 



