EVOLUTION OF PLANTS, AND DIRECTIVITY OF LIFE. 341 



In a previous lecture on the senses of plants I showed how woody 

 stems respond to strain tensions, &c., and build up mechanical struc- 

 tures to meet them. All such can become hereditary. 



Underground stems assume various forms, as tubers, conns, 

 rhizomes, creeping stems, &c. They are all adaptations for either 

 storing water, starch or other food supplies, or else for propagation ; 

 generally both purposes are combined. 



As a rule the special forms are constant in each species of plant 

 respectively, but now and then nature changes them or does not 

 develop them when the place happens to be very moist. Such is the 

 case with the grass, Poa bullosa, which has swollen internodes for 

 holding water ; similarly Ranunculus bulbosus fails to produce the 

 characteristic 11 corm " when it grows in wet and peaty soil.* The 

 leek is naturally a bulbous plant, as in the dry limestone soil of Malta, 

 where it is wild, but under cultivation the bulb is not retained, though 

 it is so in other kinds of onion (genus Allium). It occasionally 

 will reproduce the bulb. Bulbs are very characteristic of certain 

 families of monocotyledons, as the Daffodil, the Iris, and Lily 

 families. The reason appears to be that, as all monocotyle- 

 donous plants are descended from aquatic dicotyledons, when adapt- 

 ing themselves to a land situation they adopted water-storage methods 

 to enable them to live through the dry seasons. 



Long, subterranean, creeping stems of species of grasses and sedges 

 occur in sand-dunes. If a shoot of hop or mint, &c, be buried, it 

 will continue to grow for some distance and then reappear above 

 ground. The portion buried will be found to have taken on all the 

 characters of a rhizome. 



Similarly epiphytic orchids, which attach themselves to branches 

 of trees in tropical forests, by means of clasping or adhesive roots, 

 though the forest may be always wet below, the epiphytic orchids are 

 dry near to the tree-tops, so they induce their internodes to swell 

 into " pseudo-bulbs " having a very tough rind ; but the interior 

 consists of a mass of delicate cells filled with water. 



Woody-stemmed shrublets of the deserts, as near Cairo, fill their 

 cortex, or pith, &c, with water, so as to withstand the dry and hot 

 summer months. 



A result of response is often " degeneracy " — that is to say, organs 

 no longer required are supplanted by others ; the former then cease 

 to be formed or are only feebly represented. Thus aquatic stems 

 need no " wood," as the water supports them, the vascular bundles 

 degenerate, the formative " cambium " nearly or quite disappears. 

 Such is the case with all aquatic and all monocotyledonous plants. 

 In the latter the wood does not form close-fitting concentric cylinders 

 as in all timber trees, as there is no cambium layer wherewith to form 

 them. Hence palms cannot increase beyond a certain amount, radially. 

 The power to do so is lost originally by response to an aquatic life. 



* A peaty field by Poole harbour abounds with it ; but, instead of flowering 

 in May, it does not do so till August and September. 



