252 



RET. PEOF. G. HEXSLOW, M.A.^ F.L.S., 



These increase in size, till they are able to be divided in the 

 same way. Thus a " tissue " of similar cells is made. It seems 

 that the fine meridial lines of protoplasm remain and pass 

 through the new cell-wall ; so that all the cells have what is 

 called a " protoplasm continuity/' perhaps forming a sort of 

 elementary nervous system. 



How could all the preceding, and much detail is left out, be 

 done without directi^dtj ? At first the new cells are all alike : 

 but they soon have to acquire a variety of forms according to 

 the plant's requirements. Under life's directivity some will 

 elongate into spindles to make wood with thick and hard walls 

 to support the stem, others will elongate much more, and 

 instead of hardening the walls they become thick but remain 

 flexible and so form the fibre of flax and tow. Other cells 

 assume a drimi shape, one over the other in a long line. Thek 

 partitions are absorbed and a long tube or " vessel " is thus 

 made for the rapid and easy conveyance of water. To 

 strengthen these the vertical wall of the cylinder is thickened 

 in various ways ; such as by a spiral band, just as a garden hose 

 may have a coil of strong wu^e round it. 



On the exterior surface of a leaf the cells are flat, for the 

 purpose of making a skin, and if the plant grow in a very dry, 

 hot district, as a desert, the outer surface is made very thick by 

 forming a coat of substance somewhat akin to indiarubber. 

 This prevents the loss of water. On the other hand, if the 

 plant grow submerged, the skin is not wanted nor is any strong 

 supportive tissue ; so these are not formed. 



Everywhere are to be seen innumerable, purposeful arrange- 

 ments and the necessary structures to meet the necessities of 

 plant life under all conditions wherever plants can grow. 

 Without this capacity to make these adaptations, vegetable 

 and animal life would either be extremely limited or cease to 

 exist. 



This capacity is shown by the Ecsponse to the Conditions of Life ; 

 so that when seeds get dispersed and find themselves in some 

 different kind of siurroundings, and germinate, the plantlets at 

 once begin to assume new features under the " direct action of 

 the changed condition of life," as Darwin expresses it, and so 

 develop " acquired characters " in adaptation to their new 

 siuTOundings. Thus, an inland plant may acquire the fleshiness 

 of a maritime plant when growing near the sea in consequence 

 of the influence of the salt. Or a sea-side plant may become 

 quite thin-leaved if grown inland. The changes may be enough 

 to satisfy a systematic botanist that the plant can be called 



