The effects of Spirogyra on plants mainly manifest in the following aspects: 
1. Promoting Plant Growth
The aquatic algae contain various plant growth regulators, such as cytokinins and auxins. These substances can stimulate cell division and growth in plants, thereby increasing the growth rate and yield of plants. At the same time, substances like alginate, betaine, and mannitol in the aquatic algae can also provide rich nutrients for plants, further promoting their growth. 
II. Enhancing Plant Resistance
The aquatic algae grow in harsh environments, developing very strong tissues and organs that can autonomously synthesize various active substances with stress-resistant properties. These substances can enhance the plant's resistance and help it cope with various environmental pressures, such as drought, cold, and salinity. At the same time, the aquatic algae can also improve soil structure, increase soil aeration and water retention, promote the growth of beneficial microorganisms in the soil, thereby enhancing soil fertility and providing a better environment for plant growth. 
III. Enhancing Photosynthesis
The water-soluble fertilizer of Spirogyra contains a variety of essential trace elements for plant growth. These elements can facilitate the normal process of photosynthesis and enhance the photosynthetic efficiency of plants. Photosynthesis is the fundamental process of plant growth and development. By improving the photosynthetic efficiency, it can further promote the growth and development of plants. 
IV. Prevention of Nutrient Deficiency
The aquatic algae contain a rich variety of mineral elements, such as potassium, calcium, magnesium, iron, zinc, iodine, etc., totaling over 40 kinds of minerals. These elements are essential for plant growth. By applying the aquatic algae fertilizer, sufficient nutrients can be provided to the plants, preventing the occurrence of nutrient deficiency in crops. 
V. Improving Crop Quality
The algal polysaccharides and mannitol in the aquatic algae fertilizer can participate in the oxidation-reduction reactions of crops, promoting the transportation of various nutrients to the fruits, thereby improving the taste and quality of the fruits. These substances can also make the surface of the fruits smooth, increase the solid content and sugar content, extend the harvest period, increase yield and quality, and resist premature aging. 
In conclusion, the effects of Spirogyra on plants are multifaceted, including promoting plant growth, enhancing plant stress resistance, improving plant photosynthesis, preventing nutrient deficiencies, and improving crop quality. Therefore, in agricultural production, the rational use of Spirogyra fertilizer can significantly increase crop yield and quality.