rROTOPLASM : THE INSTEUMENT OF EVOLUTION AMONG PLANTS. 83 
affinity whatever between them. Similarly the Ameiican Aloe (Agave) 
of Mexico resembles, the true Aloii of Africa. The former belongs to 
the order AmaryllidecB, the latter to LiliacecE. 
Similarly with aquatic plants, a common and easily observed feature 
of submerged plants among Dicotyledons is to have the leaves finely 
dissected, by the interstitial membrane between the fibro-vascular cords 
being arrested. This occurs in such widely different plants as Water 
Crowfoot [Banunculaceoi), Cabomba (Nymphceacece), Myriophylhwi 
(Haloragece), Water Violet {FrimulacecE), Hclosciadium (Umbelliferce), 
Ceratophyllum (Ceratopthyllece) , &c. 
Here, again, the conclusion is inevitable that such a degenerated 
condition, coupled as it is with many analogous differences from leaves 
which always grow in air, is the result of living submerged. 
The stems as clearly show the effects of the action of the water, in 
having a great reduction in all the mechanical tissues required for 
supporting the stems and branches in air. Water-plants being nearly 
of the same specific gravity as water itself, they are of course not 
required ; but their use is replaced by air-canals acting as floats to 
keep the plant erect under water. 
Thus do we see how specific differences arise by the action of 
the environment upon plants, coupled with the responsive power 
possessed by protoplasm and the nucleus. 
But we may go further. It has long been recognised that all 
flowering plants may be grouped into two classes, Dicotyledons and 
Monocotyledons. Besides the difference in the presence of two cotyledons 
in the embryo of the one, and one cotyledon only in the other, the leaves 
of the former have net-veined blades, while those of the other are parallel- 
veined, allowing for a few exceptions. The wood of the stem of Dicotyle- 
dons is in concentric cylinders ; in Monocotyledons it is scattered about 
as isolated woody cords without any order. Lastly, the floral whorls are 
generally in fours or fives in the former and in threes in the latter. 
Whence arose these differences? Comparing aquatic Dicotyledons 
with Monocotyledons generally, there are a great nur^ber of distinct 
points of resemblance ; each alone might be regarded as a curious 
coincidence and nothing more ; nor do they all occur in the same 
plant ; but when taken collectively they amount to a formidable 
amount of probable evidence that Monocotyledons — though there are 
many now terrestrial — have descended from aquatic Dicotyledons. 
Thus, e.g., some aquatic plants have only one cotyledon, as the lesser 
Celandine (undoubtedly a water-plant originally) ; and the Water-chestnut 
{Trapa) is another. 
In germination the primary root of aquatic Dicotyledons is arrested, 
so that the plant is supported on secondary adventitious roots. This is 
seen in germinating Water-crowfoot, Trapa, and the Mangroves, i^c. ] t 
is universal in Monocotyledons. 
The rhizome of Water-lilies has its woody cords dispersed as in a 
Palm-tree or Asparagus shoot. 
The foliage of Water-lilies begins as phyllodes without blades, then 
follow in succession an oval blade, then a hastate, then an orbicular, and 
in the case of Nelumbmm, a peltate leaf. 
G 2 
