THE OEIGIN OF MONOCOTYLEDONS. 



291 



adventitious rootlets of dicotyledonous seeds when made to germinate 

 in water. 



The degeneracy in the anatomical structure of aquatic roots is also 

 seen in the presence of air-chambers or lacunae and in the degradation 

 of the vascular cylinder, such not being the case in plants of the same 

 species when growing on land. 



In Palms, all being terrestrial, lacunae are nevertheless cha- 

 racteristic of the roots of many species ; obviously an * ' aquatic 

 character " retained, although palms are now no longer aquatic plants. 

 Even the leaves of the banana have them. 



Proiportion of Aquatic Orders of Monocotyledons . — Of seventy-four 

 British families of Dicotyledons, five only are aquatic ; whereas of fifteen 

 British families of Monocotyledons, eight are decidedly aquatic. 

 That is to say, while 7 per cent, are aquatic among Dicotyledons, 

 53 per cent, are aquatic among Monocotyledons. 



The Effect of Water uj)on Stems. — The stems of Monocotyledons, 

 generally, reveal an internal structure, eminently characteristic of the 

 class. A transverse section shows the vascular bundles to be scattered 

 indiscriminately over the cellular ground-tissue and not arranged, 

 closely compacted, to form concentric cylinders of wood, as in all 

 timber trees among Dicotyledons. In these there is the active layer 

 just outside the wood, which forms a new cylinder every year. In 

 Monocotyledons this so-called cambium-layer is wanting, though 

 traces of it are found in various places, degraded reminiscences of a 

 former effective structure. 



This scattered condition of the bundles is well seen in terrestrial 

 Monocotyledons as palm-stems, asparagus, &c. ; it is found in the 

 rhizome of waterlilies, and in many aerial stems and petioles of aquatic 

 Dicotyledons, as described in my last lecture. 



We have seen that lacunae or air-chambers are characteristic of 

 aquatic roots and stems : and although they are not now present in 

 living palm-stems, some fossil palms have been found with 

 a-erenchyma in the stems, indicating an original aquatic habit. 

 This aerenchyma is a common feature in many Monocotyledons which 

 grow in marshy ground, but have their leaves and stems in air, as in 

 rushes, sedges, &c. It is also found in the fiower-stems of waterlilies 

 and the petioles of Rammculus Ficaria, the lesser Celandine, un- 

 doubtedly once a true aquatic plant; both of these are Dicotyledons. 



The effect of Water upon the leaves of some aquatic and other 

 plants. — The large size of the blade is a characteristic feature of certain 

 aquatic and water-loving plants. The Victoria regia, Gunnera, and 

 Rhubarb, so-called from the old name Rha (hence Rheum) of the 

 Volga, are well-known examples. Our butter-bur (Petasites) may be 

 compared with the Oolts-foot. Lastly, Palm-trees, bananas, and some 

 Aroids have enormous leaves ; a single leaf of the Talipot palm has 

 been known to shelter a whole family. The largest leaf belongs to 

 Ravenala, the Travellers' tree. The number of genera of Mono- 

 cotyledons with arborescent species is very small. These are a few 



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