592 PROGRESSIVE STAGES IN COMPLEXITY OF STRUCTURE. 



bodies. In polyp-colonies, the individual polyps i'ormed by budding remain in 

 connection with the parent-animal, and behave accordingly like the branches of a 

 compound plant-body. Yet between the parts there exists this remarkable 

 I'eciprocal relation, that the digestive cavities of individual polyps communicate 

 with one another, and that the liquids acquired by the individuals are at the 

 common service of the colony. This connection of the individual parts by com- 

 municating, sap-conducting channels also exists in plant-bodies. We call these 

 conducting channels vascular bundles, and have already had repeated occasion 

 to speak of them. They are a peculiarity of plant-hodies, and are absent in all 

 other forms of cell-unions, even in mixed systems, many of which have a great 

 resemblance to true plant-bodies, as, for example, the mosses. The difference 

 existing in this respect was the reason for placing the plants in two great divisions 

 in respect of their construction — into (1) the group of those in which vascular 

 bundles are present as architectural elements in their bodies, and (2) that in which 

 this form of cell-system is absent. The former, which are called vascular plants^ 

 form a natural group; the latter, which are called Thallophytes, are, on the other 

 hand, classed quite unsuitably. By "thallus" we understand the most different 

 vegetable structures which are devoid of vascular bundles, that is to say, not only 

 all possible tissues and systems, but also the masses of myxomycetes, even the 

 colonies and swarms of unicellular plants, structures which could not differ more 

 widely in constitution. 



It is a remarkable phenomenon that the majority of aquatic plants are devoid 

 of vascular bundles, and therefore, according to the older signification, belong to the 

 Thallophytes; and, on the other hand, that those plants which have assumed the 

 shape of plant-bodies with vascular bundles, belong almost entirely to land-plants. 

 This difference can be more accurately formulated as follows: — plants which 

 throughout their life, or at the time when they absorb nourishment, are surrounded 

 by water, saprophytes wliich are wholly imbedded in humus, and parasites 

 which are situated entirely within their hosts, absorb nourishment by the whole of 

 their superficial cells. Such structures do not require common sap-conducting 

 mechanisms, penetrating and connecting the several members. Those plants, on the 

 other hand, whose shoots are surrounded by air; which derive their fluid food from 

 the soil, and have to conduct it to the aerial organs, especially the leaves; which 

 finally conduct to the growing parts in a fluid form the organic compounds manu- 

 factured in the green tissues in sunlight; such plants require special transmitting 

 mechanisms, and as such, vascular bundles are developed in all land-plants. It is 

 necessary for the stability of the conducting mechanisms that the cells and vessels 

 in question should be lignified, or that so-called mechanical cells, i.e. hard bast, 

 should be placed near or in contact with them. Thus it is again made evident 

 that there is a difference between water-plants and land-plants in the matter 

 of rigidity. The numerous submerged plants do not possess woodj' and bast 

 cells, while these are always abundantly developed in land-plants, and to a 

 greater extent the more the plant in question requires in its natural habitat to 



