NO branch of the question to which this series of talks is devoted is more important than that with which Professor Mottram will deal this afternoon, for one may care for the growing generation in every other way and yet ruin all one's work by not understanding the elementary principles of diet. Professor Mottram, who will be remembered for his series of talks last winter, on ' Everyday Problems of Food,' is the author of 'Food and the Family,' and one of tho leading authorities on diet and health.