摘要：

Vascular plants are characterized by the existence of phloem and xylem vascular tissues that are specialized for water and nutrient transporting and mechanical support.The xylem is composed of vessels and tracheids formed by non-living cells whose cellulose cell walls are stiffened with lignin.The xylem tissues across different parts of the plant are interconnected to form a contin-uous water-conducting system.Vascular plants possess true stems,leaves,and roots,enabling them to expand to diverse habitats and to become the dominant type of land plants.How-ever,to adapt to new habitats,vascular plants,being sessile,have to face some extreme environmental conditions.Pathogens are a common and constant threat confronting plants as they migrate to new environments.To survive,plants have developed complex immune systems during an evolutionary arms race against pathogens.

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