96 CHEADLE 



plants were derived from aquatic ancestors. If this is so, then we should recog- 

 nize the essential differences between vascular plants and their aquatic an- 

 cestors and how these differences influence the ability of vascular plants to 

 live on land. Aquatic plants are bathed in water, and thus water conduction 

 throughout the plant is not of prime importance. And only when primitive 

 aquatic plants developed vegetative bodies of great length (and consequent 

 growth in deep water) was translocation of foods important. Thus we find 

 food-conducting cells — perhaps they should be considered as phloem — in the 

 stipes of such plants in the brown algae. 



But how were plants able to migrate to the land and cover the face of the 

 earth? 



To grow on land and compete for available sunlight, it is evident that the 

 structural plan of primitive aquatic plants will not do. The familiar roots and 

 leaves of our present-day plants were apparently not present in the earliest 

 vascular plants, but the available evidence indicates that they came, geo- 

 logically speaking, soon after vascular plants arose. But even from the be- 

 ginning, photosynthetic areas were carried into the air — as indeed they are 

 during ontogeny (growth from germination to maturity) of present-day plants. 

 Water had to be supplied to these food-synthesizing parts. It came from the 

 only generally available source — the soil. Water and its dissolved salts moved 

 into the underground stems of these primitive vascular plants and were un- 

 doubtedly transported rapidly upward through their xylem — just as they are 

 today. The underground anchoring and absorbing stems would have starved 

 had it not been for the translocation to them of food elaborated in the aerial 

 parts of the stem. This translocation undoubtedly took place rapidly through 

 their phloem — once again just as it does today. Our present-day plants, with 

 leaves as special food-making parts and roots as special anchoring and ab- 

 sorbing organs, have the same basic vascular tissues, modified though the 

 xylem and phloem may be. 



The development of simple but typical xylem and phloem was a triumphant 

 acquisition in the evolution of land plants. If we add to these tissues mecha- 

 nisms for conservation of water and for gaseous interchange, we have as- 

 sembled the means, as it were, for successful migration of plants from the 

 sea to a secure footing on land. 



It is undeniable, of course, that there are other factors contributing to the 

 successful migration of plants to land. It is true, likewise, that still other 

 factors are important in successful dissemination throughout the world's land 

 areas, notably the innovation of seeds and of flowers and fruits. But there 

 must be plants on land to disseminate, and it is my contention that without 

 vascular tissues there would have been no typical land plants. The loss in 

 varying degrees of vascular tissues by those derivatives of typical land plants 

 that have migrated back into the water or have become saprophytes or para- 

 sites merely underlines the validity of this contention, for they lose (and in 



